chore(db): delete tautological schema-mirror tests, collapse db singleton

- Delete 4 test files (auth/resume/agent schema mirrors + relations type check)
  that re-assert Drizzle table/column names copied verbatim from the schema files.
- Collapse makeDrizzleClient() + createDatabase() into a two-line module-level
  singleton; preserves globalThis.__drizzle caching behaviour.

Claude-Session: https://claude.ai/code/session_012Bnvt1MghwHj4qQRxuQUGa

packages/db/src/client.ts

@@ -30,15 +30,6 @@ export function getPool() {
 	return globalThis.__pool;
 }
 
-function makeDrizzleClient() {
-	return drizzle({ client: getPool(), relations });
-}
-
-function createDatabase() {
-	if (!globalThis.__drizzle) {
-		globalThis.__drizzle = makeDrizzleClient();
-	}
-	return globalThis.__drizzle;
-}
-
-export const db = createDatabase();
+// ponytail: two private fns collapsed; getPool() is already a singleton, global cache preserved
+globalThis.__drizzle ??= drizzle({ client: getPool(), relations });
+export const db = globalThis.__drizzle;