Relocate site-related types to db_types for better organization.

Changes:
- Create db_types/db_site.rs module with DbSite and DbSiteType
- Re-export types from lib.rs for backward compatibility
- Update module exports in db_types.rs

Move type-related code from mappings module to db_types for better organization.
The mappings name was misleading since we're not doing mappings anymore.

Changes:
- Move `mappings/term_relationships.rs` to `db_types/db_term_relationship.rs`
- Move `mappings/helpers.rs` to `db_types/helpers.rs`
- Create `db_types/row_ext.rs` with `ColumnIndex` and `RowExt` traits
- Update all imports throughout the codebase
- Remove old `mappings` module

Use `db_site_id: RowId` instead of `site: DbSite` in database wrapper types.
This avoids unnecessary joins since we already know the site context when querying.

Changes:
- Replace `site: DbSite` with `db_site_id: RowId` in DbAnyPostWithEditContext, DbAnyPostWithViewContext, DbAnyPostWithEmbedContext
- Replace `site: DbSite` with `db_site_id: RowId` in DbTermRelationship
- Rename column enum variants from `SiteId` to `DbSiteId` for clarity
- Update all construction and test assertions to use `db_site_id`
- Improve comment clarity for db_site_id field in DbTermRelationship

Create database structure for storing self-hosted WordPress sites with URL and API root.

Changes:
- Add `DbSelfHostedSite` struct in `db_types/self_hosted_site.rs`
- Create `0006-create-self-hosted-sites-table.sql` migration
- Modify `sites` table to add `site_type` and `mapped_site_id` columns
- Update test fixtures to properly insert into both tables
- Add migration to `MIGRATION_QUERIES` array

Add repository layer for upserting and querying self-hosted WordPress sites.

Changes:
- Create `SiteRepository` in `repository/sites.rs`
- Add table name constants for `self_hosted_sites` and `sites`
- Implement `upsert_self_hosted_site` with RETURNING optimization
- Implement `select_self_hosted_site` to query by DbSite reference
- Implement `select_self_hosted_site_by_url` to query by URL
- Add module export in `repository/mod.rs`

Create a separate domain type for self-hosted sites that doesn't include database metadata.
This improves the API design and makes it easier to add more fields in the future.

Changes:
- Add `SelfHostedSite` type with `url` and `api_root` fields
- Update `upsert_self_hosted_site` to take `&SelfHostedSite` instead of individual parameters
- Add `to_self_hosted_site()` method on `DbSelfHostedSite` for conversion

Update test fixtures to use `SiteRepository` for creating sites instead of
manually constructing `DbSite` instances. This ensures test sites are properly
inserted into the database and uses real test credentials from
`integration_test_credentials`.

Changes:
- Add `integration_test_credentials` as dev dependency
- Modify `test_db()` to return both `Connection` and `DbSite`
- Update `test_ctx()` to use actual `DbSite` from repository
- Change `create_test_site()` signature to accept `&SelfHostedSite` parameter
- Update all test call sites to create `SelfHostedSite` instances
- Use real test credentials in default test site creation

The `upsert_self_hosted_site` method had a data consistency bug where
updating an existing site URL would create orphaned entries in the
sites table. This occurred because only the self_hosted_sites table
used upsert logic while the sites table always inserted.

Changes:
- Add unique constraint on `sites(site_type, mapped_site_id)` in migration 0001
- Update `upsert_self_hosted_site` to use ON CONFLICT clause for sites table
- Ensures referential integrity between sites and self_hosted_sites tables

Implemented test suite covering all SiteRepository methods with both
positive and negative test cases. Tests use a simple fixture approach
without TestContext to avoid pollution from default site creation.

Changes:
- Rename column from `id` to `rowid` in self_hosted_sites table for consistency with posts tables
- Add `Debug`, `Clone`, `PartialEq`, `Eq` derives to `DbSelfHostedSite` for testability
- Add `count_all_sites()` method to `SiteRepository` (for testing)
- Add schema validation test for `SelfHostedSiteColumn` enum
- Add tests for `upsert_self_hosted_site` (insert and update behavior)
- Add test verifying the duplicate sites bug fix using `count_all_sites()` method
- Add tests for `select_self_hosted_site` (by DbSite reference)
- Add tests for `select_self_hosted_site_by_url`
- Add edge case tests (non-existent sites, wrong site types)
- Add test for multiple sites coexisting
- Tests use simple `test_conn` fixture instead of `TestContext`
- Tests rely on rowid equality to prove update behavior (no unnecessary counts)

All tests pass (67 total, 10 new for SiteRepository).

For consistency with the struct naming pattern where `DbSelfHostedSite`
is the database representation, the column enum should be
`DbSelfHostedSiteColumn`.

Changes:
- Rename `SelfHostedSiteColumn` to `DbSelfHostedSiteColumn`
- Update all references in `SiteRepository`
- Update test name to match new enum name

These types are only used internally by repositories and tests, not
exposed in any public API methods. Keeping them internal provides
better encapsulation.

Changes:
- Remove `pub use db_types::db_site::{DbSite, DbSiteType}` from lib.rs
- Update all modules to import directly from `crate::db_types::db_site`
- Update test modules that were using `crate::DbSiteType` to use the direct import

All tests pass (67 total).

Multi-site tests were verbose, requiring explicit URL construction for
each test site. This helper uses an internal atomic counter to generate
unique URLs automatically.

Changes:
- Add `create_random_test_site()` helper function
- Uses `RANDOM_TEST_SITE_COUNTER` static atomic to generate unique URLs
- Update `posts_multi_site_tests.rs` to use new helper
- Update `term_relationships_multi_site_tests.rs` to use new helper
- Reduces test site creation from 6 lines to 1 line

All tests pass (67 total).

Added core `delete_site()` method that handles deletion across both the
sites table and type-specific tables (self_hosted_sites). Also added
a convenience wrapper `delete_self_hosted_site_by_url()` for common use cases.

The implementation ensures proper cleanup since foreign key constraints
cannot be used with polymorphic site references. Panics with a clear
message if attempting to delete WordPress.com sites (not yet implemented).

Changes:
- Add `delete_site()` method that deletes from both tables based on site type
- Add `delete_self_hosted_site_by_url()` convenience wrapper
- Add `count_all_self_hosted_sites()` for testing
- Panic with clear message for unimplemented WordPress.com site deletion
- Add 5 comprehensive tests covering:
  - Deletion from both tables (using repository methods, no raw SQL)
  - Return value for non-existent sites
  - Deletion isolation between sites
  - URL-based deletion
  - Error cases

All tests pass (72 total, 5 new for delete functionality).

The `sites` table has been renamed to `db_sites` to match the pattern
established by `DbSite` and other database types. This makes the naming
more consistent and clear that it's a database-level polymorphic table.

Changes:
- Rename `sites` table to `db_sites` in migration 0001
- Update unique index name to `idx_db_sites_unique_site_type_and_mapped_site_id`
- Rename `SITES_TABLE` constant to `DB_SITES_TABLE`
- Update all foreign key references from `sites(id)` to `db_sites(id)` in:
  - posts_edit_context table
  - posts_view_context table
  - posts_embed_context table
  - term_relationships table
- Update all comments to reference `db_sites table` instead of `sites table`
- Rename `count_all_sites()` method to `count_all_db_sites()`
- Update all test references to use new naming

All tests pass (72 total).

Replace field-by-field comparisons with direct struct equality checks
in site repository tests, leveraging the PartialEq implementations on
DbSite and DbSelfHostedSite.

Changes:
- Use assert_eq! on structs instead of comparing individual fields
- Reduces test verbosity while maintaining same validation coverage

Standardize error handling across all tests in sites.rs by replacing unwrap() calls
with descriptive expect() messages. This improves debugging by providing clear context
when tests fail.

Changes:
- Replace unwrap() with expect() in test_conn fixture (2 calls)
- Replace unwrap() with expect() in all test functions (21 calls)
- Add descriptive error messages for database operations
- Total: 23 unwrap() calls eliminated

Ensure upsert and delete operations maintain data consistency between
the polymorphic sites table and type-specific tables by wrapping them
in atomic transactions.

Changes:
- Wrap `upsert_self_hosted_site()` in transaction to prevent orphaned entries
- Wrap `delete_site()` in transaction to ensure proper cleanup of both tables
- Scope SQL statements in blocks to ensure they drop before `tx.commit()`
- Update method signatures to use `TransactionManager` instead of `QueryExecutor`
- Update test helpers to accept `&mut Connection` for transactional operations
- Add comment explaining `DO UPDATE SET` requirement for `RETURNING` clause