feat: microservices scaffold + Auth Service (Step 1)

- Add shared/ module: config, db, models, schemas, redis utilities
- Add Auth Service (services/auth/): register, login, refresh, me,
  ForwardAuth /verify endpoint for Traefik
- Add Traefik config: ACME/Cloudflare DNS-01, dynamic routing,
  ForwardAuth middleware, sticky sessions for WS Gateway
- Add service scaffolds: ws-gateway, chat, batch-agent, billing (READMEs)
- Add redis>=5.0.0 to requirements.txt
- Monolith app/ is untouched — strangler fig migration

shared/db.py

@@ -0,0 +1,32 @@
+"""Database engine, session factory, and declarative base.
+
+All services use the async SQLAlchemy API via ``get_session()``.
+Alembic migrations use the synchronous psycopg2 URL (see alembic/env.py).
+"""
+
+from __future__ import annotations
+
+from collections.abc import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+from sqlalchemy.orm import DeclarativeBase
+
+from shared.config import settings
+
+engine = create_async_engine(
+    settings.DATABASE_URL,
+    pool_pre_ping=True,
+    echo=False,
+)
+
+async_session = async_sessionmaker(engine, expire_on_commit=False)
+
+
+class Base(DeclarativeBase):
+    """Shared declarative base for all ORM models."""
+
+
+async def get_session() -> AsyncGenerator[AsyncSession, None]:
+    """FastAPI dependency that yields an async DB session per request."""
+    async with async_session() as session:
+        yield session