feat: backend ingest API, Docker Compose, Ubuntu install guide

- FastAPI: POST /api/v1/events, GET /health, JSON Schema validation

- PostgreSQL models, Alembic migration, bootstrap API key

- deploy/docker-compose.yml, .env.example

- docs/install-ubuntu-24.04.md, updated work-plan (agents first)

backend/app/database.py

@@ -0,0 +1,22 @@
+from collections.abc import Generator
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import DeclarativeBase, Session, sessionmaker
+
+from app.config import get_settings
+
+settings = get_settings()
+engine = create_engine(settings.database_url, pool_pre_ping=True)
+SessionLocal = sessionmaker(bind=engine, autocommit=False, autoflush=False)
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+def get_db() -> Generator[Session, None, None]:
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()