feat: implement rate limiting and caching layer for MusicBrainz provider

- Add golang.org/x/time/rate dependency for token-bucket rate limiting
- Replace custom channel-based rate limiter with rate.NewLimiter(1, 1)
- Add context.Context support to doGet for cancellation
- Add cached_at column to external_releases via migration 005
- Implement cache hit/miss queries with TTL-based filtering
- Add CacheStats type for tracking cached RGIDs
- Update ExternalRelease struct with CachedAt field
- Add rate limiting tests (1 req/sec enforcement, burst behavior)
- Add cache tests (hit, miss, expired, mixed, empty artist)
- Update migration count test for new migration

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

@@ -56,13 +56,13 @@ Implement a MusicBrainz API provider with strict 1 request/second rate limiting,
 - [x] run tests - must pass before next task
 
 ### Task 2: Implement rate limiting and caching layer
-- [ ] add golang.org/x/time/rate dependency to go.mod
-- [ ] implement rate limiter using golang.org/x/time/rate.NewLimiter(1, 1) for 1 req/sec
-- [ ] create wrapper method for rate-limited HTTP GET requests
-- [ ] implement caching check: query database for existing Release Group data within TTL
-- [ ] write tests for rate limiting behavior (timing tests)
-- [ ] write tests for cache hit/miss logic
-- [ ] run tests - must pass before next task
+- [x] add golang.org/x/time/rate dependency to go.mod
+- [x] implement rate limiter using golang.org/x/time/rate.NewLimiter(1, 1) for 1 req/sec
+- [x] create wrapper method for rate-limited HTTP GET requests
+- [x] implement caching check: query database for existing Release Group data within TTL
+- [x] write tests for rate limiting behavior (timing tests)
+- [x] write tests for cache hit/miss logic
+- [x] run tests - must pass before next task
 
 ### Task 3: Implement MusicBrainz API endpoints and filtering
 - [ ] implement GetArtistReleaseGroups(artistMBID string) method