refactor: pull fn into_utc into atuin-server-database crate

[oxo42]

The fn into_utc was defined in both atuin-server-postgres and atuin-server-sqlite, with tests being in the postgres crate. This pulls the function into the atuin-server-database crate along with the tests and references it from both crates.

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[greptile-apps]

Greptile Summary

This PR moves the shared UTC timestamp conversion into the database crate. The main changes are:

• Adds into_utc to atuin-server-database.
• Moves the existing UTC conversion tests with the helper.
• Updates PostgreSQL and SQLite backends to import the shared helper.
• Removes the duplicate backend-local helper definitions.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The moved helper keeps the same signature and conversion behavior.
• The backend call sites still bind the same timestamp type as before.

Important Files Changed

Filename	Overview
crates/atuin-server-database/src/lib.rs	Adds the shared into_utc helper and its existing conversion tests.
crates/atuin-server-postgres/src/lib.rs	Replaces the local timestamp conversion helper with the shared database-crate helper.
crates/atuin-server-sqlite/src/lib.rs	Replaces the local timestamp conversion helper with the shared database-crate helper.

_{Reviews (1): Last reviewed commit: "refactor: pull into_utc into atuin-serve..." | Re-trigger Greptile}

[ellie]

thank you!!