feat(sites): cross-process site rate limit via PG token bucket

Migration 0041_rate_limit adds rate_limit_bucket. PgTokenBucket (@umt/db)
refills and takes tokens under SELECT ... FOR UPDATE on the DB clock and
polls with jitter within a wait budget; penalize() carries Retry-After.
Site adapters get createSharedSiteRateLimiter (key site:<id>), shared by
api / worker / worker:agent, with an in-process serial queue in front and
a local fallback when the bucket is unavailable.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/db/src/index.ts

@@ -2,4 +2,11 @@
 export { type Kysely, sql, type Transaction } from "kysely";
 export { type CreateDbOptions, createDb } from "./client.ts";
 export { createMigrator, MIGRATIONS_DIR, type MigratorOptions, migrateToLatest, resetSchema } from "./migrator.ts";
+export {
+  type AcquireOptions,
+  type AcquireResult,
+  type BucketSpec,
+  PgTokenBucket,
+  pgTokenBucketFor,
+} from "./rate-limit.ts";
 export type * from "./types.ts";