[pournasserian]

Closes #2451

What

The second slice of F-SCH (#2415): content types are stored and read back through an application service. This builds on the Core model from #2445.

• Core:
  • Site holds the one well-known default site (SCH-07).
  • IContentTypeDirectory lists types, gets the current version, gets a given version, creates and revises, each returning a Result. A revision names the version it starts from.
  • ContentType.Restore rebuilds a stored type without validating it again, so a type stays readable after a field type it uses is gone.
  • IFieldType.ReferencedTypes (empty by default) lets a field type declare the content types it points at. reference returns its target, and list asks its item type through the registry.
• Infrastructure:
  • Sites is seeded with the default site by the migration.
  • ContentTypes holds the site, name, slug and current version number, with (SiteId, Slug) unique.
  • ContentTypeVersions holds one row per version with the definition JSON, jsonb on PostgreSQL. (ContentTypeId, Version) is unique, and rows are inserted once and never changed.
  • ContentTypeDirectory checks slug uniqueness and reference targets against stored data. It writes through IUnitOfWork and appends content_type.created or content_type.revised through IAuditLog, with the content type as subject. The revised event's PreviousVersionId is the replaced version's row id.
• Migrations ContentTypes for both providers, generated with the documented commands.
• CONTRIBUTING.md: how a field type declares the types it references.

Why

SCH-01 and SCH-07 need the types stored and scoped to a site. The epic's acceptance needs earlier versions to stay readable. AUD-N1 needs every schema write audited in the same transaction.

Some choices worth a look:

• Site: one row, seeded by the migration, with a fixed id in Core. Every type carries it and a foreign key enforces it. The directory scopes every read and write to it. Nothing else about sites exists yet, so MST-01 can decide how a caller picks a site.
• Expected failures before the unit of work: the directory runs its checks before it opens its own scope. Inside a scope, an early return would dispose an uncommitted inner scope and doom the caller's outer unit of work. A test covers a failed create inside an outer unit of work that still commits.
• Concurrency: two writers can both pass a check before either commits. A create then trips the (SiteId, Slug) index, and the directory maps that unique violation to the same slug_taken result. A revision moves the version pointer with UPDATE … WHERE Version = @base, so the second writer updates no row and gets version_conflict. The (ContentTypeId, Version) index backs this up. The mapping recognises SQLite SQLITE_CONSTRAINT_UNIQUE/PRIMARYKEY and PostgreSQL 23505.
• No tracking on reads: a long-lived context, such as a Blazor circuit, never sees a stale tracked row.
• Reference checks go through IFieldType.ReferencedTypes, not a special case for the built-in reference type. A plugin type that points at entries gets the same check (EXT-N1). This adds a defaulted member to the field type interface. Existing implementations are unaffected.
• Test helpers: the integration tests use TemporaryDatabase and PostgresDataAttribute from Add reading and JSON and CSV export of the audit log #2449 to run every case on both providers.

Tests

Unit:

• reference declares its target. list declares its item type's targets.
• A definition lists each referencing field, including a plugin type's.
• The conformance base gains "declares only slugs as referenced types", which all built-ins and both sample plugin types pass.
• Restore keeps stored values without validating them.

Integration, each on SQLite and PostgreSQL:

• A created type reads back in the default site at version 1 with an audit event naming the type and the initiator.
• A revision stores version 2, version 1 stays readable, and the revised event points at the replaced version's row.
• A taken slug, an unknown reference target, an invalid definition and a stale or future base version each fail, write nothing and append no event. A self-reference is allowed.
• A create inside an abandoned unit of work leaves nothing.
• A failed create inside the caller's unit of work leaves it able to commit.
• Two concurrent creates of one slug and two concurrent revisions of one version each end with one winner and a failed Result. Disabling the unique-violation mapping makes the create test fail with DbUpdateException on both providers, and disabling the conditional update too makes the revision test fail, so both tests reach the database path.
• The reference scenario's eight types can be defined.

PostgreSQL only: the definition column is jsonb. The existing migration test reverts the newest migration (ContentTypes) and reapplies it on SQLite.

All of this ran locally against a postgres:17 container as well as SQLite. The schema tests passed five runs in a row.

No new package and no new project reference.

Checklist

• Linked to an issue a maintainer has confirmed
• Targets v2
• Tests or docs updated where behaviour changed
• No tool-generated co-author trailers or footers