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>

.env.example

@@ -13,6 +13,17 @@ DB_USER=goodgo
 DB_PASSWORD=CHANGE_ME
 DATABASE_URL=postgresql://${DB_USER}:${DB_PASSWORD}@${DB_HOST}:${DB_PORT}/${DB_NAME}?schema=public
 
+# Direct connection (bypasses PgBouncer — used for migrations/introspection)
+DATABASE_URL_DIRECT=postgresql://${DB_USER}:${DB_PASSWORD}@${DB_HOST}:${DB_PORT}/${DB_NAME}?schema=public
+
+# -----------------------------------------------------------------------------
+# PgBouncer (Connection Pooling — production only)
+# -----------------------------------------------------------------------------
+PGBOUNCER_POOL_SIZE=20
+PGBOUNCER_MAX_CLIENT_CONN=200
+PGBOUNCER_ADMIN_PASSWORD=CHANGE_ME
+PGBOUNCER_STATS_PASSWORD=CHANGE_ME
+
 # -----------------------------------------------------------------------------
 # Redis
 # -----------------------------------------------------------------------------