perf(S01/T01): Add token-bucket rate limiter (1 req/sec), SQLite respon…

- backend/explore/ratelimiter.go
- backend/explore/cache.go
- backend/database/sql/schemas/explore_cache.sql
- backend/database/database.go

backend/explore/ratelimiter.go

@@ -0,0 +1,37 @@
+// Package explore provides MusicBrainz and ListenBrainz API clients
+// with rate-limited HTTP access and a SQLite response cache.
+package explore
+
+import (
+	"context"
+	"time"
+
+	"golang.org/x/time/rate"
+)
+
+// RateLimiter enforces a maximum request rate using a token bucket.
+// MusicBrainz requires ≤1 request per second and rejects ALL
+// requests (not just excess) when the rate is exceeded, so callers
+// block proactively via Wait rather than retrying reactively.
+//
+// RateLimiter is safe for concurrent use.
+type RateLimiter struct {
+	limiter *rate.Limiter
+}
+
+// NewRateLimiter returns a rate limiter that allows exactly one
+// request per second with a burst size of 1. The first call to
+// Wait returns immediately; subsequent calls block until the next
+// token is available.
+func NewRateLimiter() *RateLimiter {
+	return &RateLimiter{
+		limiter: rate.NewLimiter(rate.Every(time.Second), 1),
+	}
+}
+
+// Wait blocks until the rate limiter allows the caller to proceed
+// or the context is cancelled. Returns ctx.Err() if the context
+// expires before a token becomes available.
+func (r *RateLimiter) Wait(ctx context.Context) error {
+	return r.limiter.Wait(ctx)
+}