feat(db): Postgres schema, migrations and defects repository (T3)

Drizzle schema for projects, defects, push_subscriptions (C8); generated
migration; a defects repository (createDefect / listRecentDefects); a
programmatic migrator; and a db:verify script. section_id is a plain string
referencing in-code board ids, no FK (ADR 0001). Verified against real
Postgres: migration applies, a defect round-trips, /api/health reports db up.

server/db/migrate.ts

@@ -0,0 +1,7 @@
+// Applies pending migrations. Run via `pnpm db:migrate`, and on container
+// boot in production (wired in Task 12).
+import { migrate } from 'drizzle-orm/node-postgres/migrator'
+import { getDb, getPool } from './client'
+
+await migrate(getDb(), { migrationsFolder: 'server/db/migrations' })
+await getPool().end()