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

go.sum

@@ -2,6 +2,8 @@ github.com/delucks/go-subsonic v0.0.0-20240806025900-2a743ec36238 h1:uejyepOdHIS
 github.com/delucks/go-subsonic v0.0.0-20240806025900-2a743ec36238/go.mod h1:vnbEuj6Z20PLcHB4rrLQAOXGMjtULfMGhRVSFPcSdUo=
 github.com/mattn/go-sqlite3 v1.14.22 h1:2gZY6PC6kBnID23Tichd1K+Z0oS6nE/XwU+Vz/5o4kU=
 github.com/mattn/go-sqlite3 v1.14.22/go.mod h1:Uh1q+B4BYcTPb+yiD3kU8Ct7aC0hY9fxUwlHK0RXw+Y=
+golang.org/x/time v0.15.0 h1:bbrp8t3bGUeFOx08pvsMYRTCVSMk89u4tKbNOZbp88U=
+golang.org/x/time v0.15.0/go.mod h1:Y4YMaQmXwGQZoFaVFk4YpCt4FLQMYKZe9oeV/f4MSno=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=