Collapse the three collection endpoint modules into one generic owner-scoped module

[mforce]

What to build

Collapse the three collection endpoint modules (movies, music, games) into one generic owner-scoped collection endpoint module plus a thin per-type configuration.

The three endpoint files are 75–78% identical after renaming the entity token — the CRUD skeleton for movies vs games differs by zero lines. Auth, ownership scoping (hand-written in every handler), the page cap, tag filtering, validation strings, and cover/tag resolution are each re-declared per type; roughly 609 lines carry about 150 lines of genuinely distinct behavior. The distinct part is exactly: per-type filter parameters (movie format bitmask test, music format equality, game platform / digital / completion status), per-type DTO shape, and the game-specific PlatformLegacy clearing on update.

Build one generic module implementing list/get/create/update/delete with ownership scoping, paging, tag filtering, validation, and cover/tag resolution once, parameterized by a per-type config (DTO mapping, validator, filter strategy). Movies/Music/Games become declarations. Wire contracts must not change — routes, DTO shapes (including the movie Formats integer bitmask), status codes, and validation messages stay identical, which the parameterized endpoint test suite pins.

Acceptance criteria

• Auth, ownership scoping, page cap, tag filtering, validation, and mapping are implemented once
• Per-type filters and the PlatformLegacy clearing behavior are preserved via config
• Wire contracts unchanged: the parameterized endpoint tests pass without modification
• Adding a media type requires an entity plus a config declaration, not a new 200-line endpoint file

Blocked by

• Parameterize collection endpoint tests across media types, consolidate test infrastructure #96 — the parameterized test suite is the safety net for this refactor
• Recommended after Unify metadata provider interfaces into generic IMetadataProvider<T>, delete pass-through adapters #94 (provider-interface cleanup) but not strictly blocked by it

Added scope — F2: reflection guard that every write-boundary enum has a registered converter

Carried over from PR #141 review (issue #115 follow-up). Folded into this issue so the generic module ships with it from day one.

Problem: Program.cs hand-registers six enums (CollectionStatus, Condition, WatchStatus, CompletionStatus, DigitalStore, MusicFormat) with DefinedEnumConverter<TEnum>; GamePlatform gets GamePlatformJsonConverter; MovieFormat is an int bitmask validated in the module. Nothing today catches a forgotten registration. Under #97's "add a media type via config declaration, not a 200-line endpoint file" model, an author who adds a media type and omits the config declaration silently falls back to the permissive default converter — re-opening the hole #115 closed, with no test failure to warn.

Requirement (red-first TDD): a reflection test enumerates every concrete enum appearing in write DTO / endpoint bodies, and asserts the test's JsonSerializerOptions (mirroring Program.cs) has a registered converter (DefinedEnumConverter<TEnum> or GamePlatformJsonConverter). A brand-new enum with no registration must fail this named test with an actionable message naming the enum and the missing declaration (mutation-checked). MovieFormat is excluded (int bitmask, validated in the module); GamePlatform counts as covered via its dedicated converter. Non-goals (separate issues): F4 legacy-row heal path, 400-body normalization.