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

internal/musicbrainz/cache.go

@@ -0,0 +1,53 @@
+package musicbrainz
+
+import (
+	"fmt"
+	"time"
+
+	"naviwatcher/internal/database"
+)
+
+// CacheStats holds the result of a cache lookup for a given artist.
+type CacheStats struct {
+	// CachedRGIDs is the list of RGIDs that are currently cached (within TTL).
+	CachedRGIDs []string
+	// CacheHitCount is the number of entries found in cache.
+	CacheHitCount int
+}
+
+// IsCached returns true if the given RGID is in the cached set.
+func (cs *CacheStats) IsCached(rgid string) bool {
+	for _, id := range cs.CachedRGIDs {
+		if id == rgid {
+			return true
+		}
+	}
+	return false
+}
+
+// GetCachedReleases queries the external_releases table for entries
+// belonging to the given artist that were cached within the specified TTL.
+// It returns a CacheStats with the list of valid RGIDs already in cache.
+func GetCachedReleases(db *database.DB, artistID string, ttl time.Duration) (*CacheStats, error) {
+	releases, err := database.GetExternalReleasesByArtistWithCache(db, artistID, ttl)
+	if err != nil {
+		return nil, fmt.Errorf("get cached releases: %w", err)
+	}
+
+	stats := &CacheStats{}
+	for _, r := range releases {
+		stats.CachedRGIDs = append(stats.CachedRGIDs, r.RGID)
+		stats.CacheHitCount++
+	}
+	return stats, nil
+}
+
+// IsArtistCacheValid checks whether the cache for an artist is still valid.
+// Returns true if any entries exist within the TTL for this artist.
+func IsArtistCacheValid(db *database.DB, artistID string, ttl time.Duration) (bool, error) {
+	stats, err := GetCachedReleases(db, artistID, ttl)
+	if err != nil {
+		return false, err
+	}
+	return stats.CacheHitCount > 0, nil
+}