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/api/v1/health.py

@@ -0,0 +1,21 @@
+from fastapi import APIRouter, Depends
+from sqlalchemy import text
+from sqlalchemy.orm import Session
+
+from app.database import get_db
+
+router = APIRouter(tags=["health"])
+
+
+@router.get("/health")
+def health(db: Session = Depends(get_db)) -> dict:
+    db_status = "ok"
+    try:
+        db.execute(text("SELECT 1"))
+    except Exception:
+        db_status = "error"
+    return {
+        "status": "ok" if db_status == "ok" else "degraded",
+        "database": db_status,
+        "service": "security-alert-center",
+    }