feat(db): 统一元数据缓存层 MetadataCache

- 两级:L1 进程内存(按 region 限条数的 LRU、5 分钟)+ L2 PG kv_cache;读 L1 → L2 → 源,写两级
- 迁移 0086:kv_cache 加 fresh_until(过期先用旧值的宽限期)与 pinned(LRU 淘汰跳过);新表 cache_stat 按天 × region 汇总命中
- stale-while-revalidate:宽限期 = TTL 的 50%(上限 7 天),旧值先返回、同 key 后台只刷一次,刷新失败继续用旧值并记警告
- 同进程同 key 并发只回源一次;回源抛错不缓存;PG 读写失败按未命中 / 不写处理
- PgCache.delete / clear 发 pg_notify(umt_cache_invalidate),listenCacheInvalidation 收到后丢 L1;监听在线才启用 L1,断线停用、重连后整体清空
- PgCache.get 只认新鲜条目,getEntry 带回旧值;renew 只续新鲜条目并可标 pinned

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Cxwgnn2pbzQ7rXv3rq7fei

packages/db/src/index.ts

@@ -1,13 +1,24 @@
 // 统一从本包取 Kysely 与 sql,避免各包各装一份 kysely
 export { type Kysely, type RawBuilder, type Selectable, sql, type Transaction } from "kysely";
 export {
+  CACHE_INVALIDATE_CHANNEL,
+  type CacheCounters,
   type CachedOptions,
+  type CacheEntry,
   cachedCall,
   DEFAULT_CACHE_SIZE,
   DEFAULT_CACHE_TTL,
+  defaultGrace,
+  type FetchPolicy,
+  listenCacheInvalidation,
+  MetadataCache,
+  type MetadataCacheOptions,
+  metadataCacheFor,
   PgCache,
   pgCacheFor,
   type SetOptions as CacheSetOptions,
+  STALE_GRACE_MAX_SEC,
+  STALE_GRACE_RATIO,
 } from "./cache.ts";
 export { type CreateDbOptions, createDb } from "./client.ts";
 export { createMigrator, MIGRATIONS_DIR, type MigratorOptions, migrateToLatest, resetSchema } from "./migrator.ts";