feat: implement MusicBrainz sync orchestration with caching and upsert

Creates internal/musicbrainz/sync.go with SyncArtistDiscography function
that orchestrates the full flow: cache check via GetCachedReleases, fetch
from MusicBrainz API on cache miss, filter via FilterReleaseGroups, and
upsert into external_releases via database.SaveExternalRelease. Includes
SyncArtistDiscographyWithFilter variant for per-artist type filtering. All
functions support context cancellation. 14 new tests cover cache hit/miss,
status/type filtering, context cancellation, idempotency, API errors, and
full XML pipeline integration.

docs/plans/2026-05-21-musicbrainz-provider.md

@@ -75,13 +75,13 @@ Implement a MusicBrainz API provider with strict 1 request/second rate limiting,
 - [x] run tests - must pass before next task
 
 ### Task 4: Implement database integration and sync orchestration
-- [ ] create `internal/musicbrainz/sync.go` with SyncArtistDiscography function
-- [ ] implement upsert logic: INSERT OR REPLACE into external_releases table
-- [ ] add cached_at column to external_releases table via migration
-- [ ] implement context.Context support for cancellation
-- [ ] write tests for database upsert operations
-- [ ] write integration tests with in-memory SQLite
-- [ ] run tests - must pass before next task
+- [x] create `internal/musicbrainz/sync.go` with SyncArtistDiscography function
+- [x] implement upsert logic: INSERT OR REPLACE into external_releases table
+- [x] add cached_at column to external_releases table via migration (done in Task 3 as migration 005)
+- [x] implement context.Context support for cancellation
+- [x] write tests for database upsert operations
+- [x] write integration tests with in-memory SQLite
+- [x] run tests - must pass before next task
 
 ### Task 5: Wire up provider in application entry point
 - [ ] update `cmd/naviwatcher/main.go` to initialize MusicBrainz client