M1: backend core + proposal engine

- FastAPI skeleton, SQLAlchemy models (§13), Alembic initial migration
- SchedulingProvider interface with google_calendar (free/busy read-only),
  partner_api (Appendix B client) and mock implementations
- Proposal engine: create → provider-routed delivery → owner actions
  (resolve/confirm+SMS/reject) → expiry + reminders (§9)
- Signed single-use action links, .ics METHOD:REQUEST attachment
- Partner outcome webhook with HMAC verification + polling fallback
- SmsProvider (console) with Bosnian templates (§5.5), EmailProvider (console/SMTP)
- Fake partner API server in tests/ — Appendix B reference implementation
- 43 tests: slot math, proposal lifecycle, action links, partner contract

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

gogo/db.py

@@ -0,0 +1,43 @@
+"""Async SQLAlchemy engine/session setup."""
+
+from collections.abc import AsyncIterator
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+from sqlalchemy.orm import DeclarativeBase
+
+from gogo.config import get_settings
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+_engine = None
+_sessionmaker: async_sessionmaker[AsyncSession] | None = None
+
+
+def get_engine():
+    global _engine, _sessionmaker
+    if _engine is None:
+        _engine = create_async_engine(get_settings().database_url, pool_pre_ping=True)
+        _sessionmaker = async_sessionmaker(_engine, expire_on_commit=False)
+    return _engine
+
+
+def get_sessionmaker() -> async_sessionmaker[AsyncSession]:
+    get_engine()
+    assert _sessionmaker is not None
+    return _sessionmaker
+
+
+async def get_session() -> AsyncIterator[AsyncSession]:
+    """FastAPI dependency."""
+    async with get_sessionmaker()() as session:
+        yield session
+
+
+def reset_engine() -> None:
+    """Test helper: force re-creation of the engine (e.g. after settings change)."""
+    global _engine, _sessionmaker
+    _engine = None
+    _sessionmaker = None