feat(infra): add PgBouncer connection pooling for production PostgreSQL

Introduces PgBouncer as a connection pooler between the API service and
PostgreSQL in docker-compose.prod.yml, reducing connection overhead and
improving concurrency under production load.

- Add PgBouncer service (edoburu/pgbouncer:1.23.1-p2) with transaction
  pool mode, max_client_conn=200, default_pool_size=20
- Route API DATABASE_URL through PgBouncer (port 6432), keep direct
  connection (DATABASE_URL_DIRECT) for Prisma migrations/introspection
- Create infra/pgbouncer/ config: pgbouncer.ini, userlist template,
  and entrypoint script with runtime env-var substitution
- Update prisma.config.ts to prefer DATABASE_URL_DIRECT for migrations
- Add K6 load test (e2e/load/pgbouncer-pool-test.js) with ramp-up to
  200 VUs, pool exhaustion detection, and p95 < 2s threshold
- Add PgBouncer env vars to .env.example

Co-Authored-By: Paperclip <noreply@paperclip.ing>

prisma/prisma.config.ts

@@ -6,8 +6,10 @@ export default defineConfig({
   schema: path.join(__dirname, 'schema.prisma'),
   migrate: {
     async development() {
+      // Use DATABASE_URL_DIRECT (bypasses PgBouncer) for migrations/introspection
+      // when available; fall back to DATABASE_URL for local dev without PgBouncer.
       return {
-        url: process.env.DATABASE_URL!,
+        url: process.env.DATABASE_URL_DIRECT || process.env.DATABASE_URL!,
       };
     },
   },