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/database/sql/schemas/explore_cache.sql

@@ -0,0 +1,10 @@
+CREATE TABLE IF NOT EXISTS explore_cache (
+    url_key     TEXT PRIMARY KEY,
+    response    TEXT NOT NULL,
+    mbid        TEXT,
+    entity_type TEXT,
+    expires_at  DATETIME NOT NULL,
+    created_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX IF NOT EXISTS idx_explore_cache_expires ON explore_cache(expires_at);
+CREATE INDEX IF NOT EXISTS idx_explore_cache_mbid ON explore_cache(mbid);