P2 infra: add PgBouncer before Postgres

## Phase / Priority / Area
Phase 15 · Priority P2 · Area infra

## Scope
`deploy/docker-compose.yml`, `deploy/pgbouncer/pgbouncer.ini` (new), `.env.example`, `services/auth/src/core/db.py:26`

## Goal
Пул коннектов для N видео.

## Acceptance Criteria
- [ ] `pgbouncer :6432` перед `postgres:5432`, `pool_size` из env

## Проверки
`psql via 6432`, `show pools`
