Run db:migrate automatically before every Railway deploy

The 2026-07-06 prod outage was a missed manual migration: the 0002
job_events migration never ran against prod, so the worker crash-looped
on every job. Make the deploy pipeline own it:

- railway.json sets `npm run db:migrate` as the pre-deploy command.
  Railway runs it with the service's env before starting the new
  deployment; a failed migration fails the deploy and the old version
  keeps serving.
- runMigrations() now serializes on a Postgres advisory lock so api and
  worker pre-deploys firing off the same push can't race Drizzle's
  journal writes.
- DEPLOY.md: document the automatic path; keep the manual command for
  first-time setup.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/db/src/migrate.ts

@@ -6,12 +6,26 @@ import { createDb } from "./client.js";
 const HERE = dirname(fileURLToPath(import.meta.url));
 export const MIGRATIONS_DIR = join(HERE, "..", "migrations");
 
-/** Apply all pending Drizzle migrations to the target database. */
+/** App-wide advisory lock id for migrations. Concurrent `db:migrate` runs (e.g.
+ *  api + worker pre-deploy firing off the same push) serialize on it instead of
+ *  racing Drizzle's journal writes. */
+const MIGRATE_LOCK_ID = 0xd1770;
+
+/** Apply all pending Drizzle migrations to the target database. Safe to run
+ *  concurrently: callers serialize on a Postgres advisory lock, and re-running
+ *  applied migrations is a journal no-op. */
 export async function runMigrations(connectionString: string, migrationsFolder = MIGRATIONS_DIR): Promise<void> {
   const { db, pool } = createDb(connectionString);
+  const client = await pool.connect();
   try {
-    await migrate(db, { migrationsFolder });
+    await client.query("SELECT pg_advisory_lock($1)", [MIGRATE_LOCK_ID]);
+    try {
+      await migrate(db, { migrationsFolder });
+    } finally {
+      await client.query("SELECT pg_advisory_unlock($1)", [MIGRATE_LOCK_ID]);
+    }
   } finally {
+    client.release();
     await pool.end();
   }
 }