Fix on-the-fly event store schema + InitialData seeding on startup (#219)

[jeremydmiller]

Closes #219.

Analysis: two gaps, both "Polecat doesn't evaluate schema on first use like Marten does"

The reporter found that IInitialData seeding only ran when ApplyAllDatabaseChangesOnStartup was called. Digging in (per the maintainer's framing — on-the-fly migration for the first usage of a document type or the IEvent) surfaced two distinct gaps:

1. The event store is not created on the fly ❌ → ✅

Document tables ARE ensured on first Store/query (DocumentTableEnsurer), and document queries ensure too — but the event store tables (pc_streams / pc_events / pc_event_progression, plus tag and natural-key tables) were only ever created by ApplyAllDatabaseChangesOnStartup. A first append or event query on a fresh DB failed:

Invalid object name 'pc_streams'.   -- on append
Invalid object name 'pc_events'.    -- on FetchStreamAsync

(Confirmed with a failing repro before the fix.)

Fix: added DocumentTableEnsurer.EnsureEventStoreSchemaAsync (idempotent, once per process, applies the EventStoreFeatureSchema) and call it from:

• the append path — DocumentSessionBase.SaveChanges, before the data transaction, only when there are events to write (it opens its own connection, like the document ensure);
• the event read path — QueryEventStore's leaf helpers (FetchStream/Load/FetchStreamState; the aggregate helpers delegate through these), via a new QuerySession.EnsureEventStoreSchemaAsync.

2. InitialData never ran without ApplyAllDatabaseChangesOnStartup ❌ → ✅

The PolecatActivator hosted service that runs IInitialData.Populate was only registered by ApplyAllDatabaseChangesOnStartup / AddAsyncDaemon / AddProjectionCoordinator. So the documented pattern —

builder.Services.AddPolecat(opts => opts.InitialData.Add(new SeedUsers()));

— silently did nothing (the reporter's exact symptom). Fix: AddPolecat now registers the activator unconditionally. Its StartAsync is a no-op when there's no InitialData and ShouldApplyChangesOnStartup is false, so it's safe for every app. The seeder writes through normal sessions, so the document tables it touches are still created on the fly.

Also: honor AutoCreate.None

The maintainer's principle was "on the fly unless AutoCreate.None." The document ensurer actually ignored AutoCreate.None and always created tables. Both the document and the new event on-the-fly paths now skip implicit creation under AutoCreate.None, so the manual-schema opt-out works as intended.

Tests

• on_the_fly_event_store_tests — appending and querying events on a fresh DB now create the event store; AutoCreate.None correctly skips (append throws).
• initial_data_host_startup_tests — builds a real service provider with AddPolecat(... InitialData.Add ...) and no ApplyAllDatabaseChangesOnStartup, starts the hosted services, and confirms the seeder ran — using marcominerva's query-then-seed shape (which also exercises on-the-fly document creation from inside the seeder).

Verified on SQL Server 2025 and 2022; broad Seeding/Events/Storage/Documents/Projections/MultiTenancy/Diagnostics suites green (~600 tests, including all existing AutoCreate.None tests).

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