refactor(db): consolidate the two Postgres pools

db/client.ts was creating its own pg Pool with its own lazy
initialization, while src/lib/db.ts already had an identical pool
and a withTx helper. Two pools per process means double the Postgres
connections, double the env-var parsing, and double the BEGIN/COMMIT
plumbing to maintain.

After this change:
- db/client.ts imports getPool/withTx from src/lib/db.ts
- withBrand and withPlatformAdmin wrap withTx (single transaction
  helper)
- One pg.Pool per Node process

No behavior change for callers — withTx is byte-identical to the
deleted runInTransaction. The shared getPool uses a 30s connection
timeout (was 10s in the deleted copy); 30s is what src/lib/db.ts has
always shipped to production via its callers, so this aligns the
Drizzle layer with the rest of the app.

db/client.ts

@@ -23,38 +23,22 @@
  */
 
 import "server-only";
-import { Pool, type PoolClient } from "pg";
 import { drizzle, type NodePgDatabase } from "drizzle-orm/node-postgres";
 import * as schema from "./schema";
+import { getPool, withTx } from "@/lib/db";
 
 type Schema = typeof schema;
 export type Db = NodePgDatabase<Schema>;
 
-let _pool: Pool | null = null;
-
-function getPool(): Pool {
-  if (_pool) return _pool;
-  const connectionString = process.env.DATABASE_URL;
-  if (!connectionString) {
-    throw new Error(
-      "DATABASE_URL is not set. Add it to .env.local (see .env.example).",
-    );
-  }
-  _pool = new Pool({
-    connectionString,
-    max: parseInt(process.env.PG_POOL_MAX ?? "10", 10),
-    idleTimeoutMillis: parseInt(process.env.PG_POOL_IDLE_MS ?? "30000", 10),
-    connectionTimeoutMillis: parseInt(
-      process.env.PG_POOL_CONN_TIMEOUT_MS ?? "10000",
-      10,
-    ),
-    allowExitOnIdle: false,
-  });
-  _pool.on("error", (err) => {
-    console.error("[db] idle client error", err);
-  });
-  return _pool;
-}
+/**
+ * The Drizzle layer reuses the shared `pg` `Pool` from `@/lib/db` so the
+ * app connects to Postgres through a single pool. Connection settings
+ * (`PG_POOL_MAX`, `PG_POOL_IDLE_MS`, `PG_POOL_CONN_TIMEOUT_MS`,
+ * `DATABASE_URL`) live in one place. See `src/lib/db.ts` for the config.
+ *
+ * The `withBrand` and `withPlatformAdmin` helpers reuse `withTx` from
+ * `@/lib/db` so the BEGIN/COMMIT/ROLLBACK handling has a single home.
+ */
 
 /**
  * Run `fn` with a Drizzle client. No brand context is set — the caller
@@ -83,7 +67,7 @@ export async function withBrand<T>(
   brandId: string,
   fn: (db: Db) => Promise<T>,
 ): Promise<T> {
-  return runInTransaction(async (client) => {
+  return withTx(async (client) => {
     // set_config(setting, value, is_local) — is_local=true makes it
     // transaction-local so it auto-resets at COMMIT/ROLLBACK and never
     // leaks across pooled connections.
@@ -103,7 +87,7 @@ export async function withBrand<T>(
 export async function withPlatformAdmin<T>(
   fn: (db: Db) => Promise<T>,
 ): Promise<T> {
-  return runInTransaction(async (client) => {
+  return withTx(async (client) => {
     await client.query("SELECT set_config('app.current_brand_id', '', true)");
     await client.query("SELECT set_config('app.platform_admin', 'true', true)");
     const db = drizzle(client, { schema });
@@ -111,25 +95,4 @@ export async function withPlatformAdmin<T>(
   });
 }
 
-async function runInTransaction<T>(
-  fn: (client: PoolClient) => Promise<T>,
-): Promise<T> {
-  const client = await getPool().connect();
-  try {
-    await client.query("BEGIN");
-    const result = await fn(client);
-    await client.query("COMMIT");
-    return result;
-  } catch (err) {
-    try {
-      await client.query("ROLLBACK");
-    } catch {
-      // ignore secondary rollback failure
-    }
-    throw err;
-  } finally {
-    client.release();
-  }
-}
-
 export { schema };