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/main.py

@@ -0,0 +1,66 @@
+from contextlib import asynccontextmanager
+
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+from sqlalchemy import select
+
+from app.api.v1.router import api_router
+from app.auth.api_key import hash_api_key
+from app.config import get_settings
+from app.database import SessionLocal
+from app.models import ApiKey
+
+
+def bootstrap_api_key() -> None:
+    settings = get_settings()
+    if not settings.sac_bootstrap_api_key:
+        return
+    db = SessionLocal()
+    try:
+        key_hash = hash_api_key(settings.sac_bootstrap_api_key)
+        exists = db.scalar(select(ApiKey).where(ApiKey.key_hash == key_hash))
+        if exists:
+            return
+        prefix = settings.sac_bootstrap_api_key[:12]
+        db.add(
+            ApiKey(
+                name="bootstrap",
+                key_prefix=prefix,
+                key_hash=key_hash,
+                is_active=True,
+            )
+        )
+        db.commit()
+    finally:
+        db.close()
+
+
+@asynccontextmanager
+async def lifespan(_app: FastAPI):
+    bootstrap_api_key()
+    yield
+
+
+def create_app() -> FastAPI:
+    settings = get_settings()
+    app = FastAPI(
+        title="Security Alert Center",
+        version="0.1.0",
+        lifespan=lifespan,
+    )
+    origins = [o.strip() for o in settings.cors_origins.split(",") if o.strip()]
+    app.add_middleware(
+        CORSMiddleware,
+        allow_origins=origins if origins != ["*"] else ["*"],
+        allow_credentials=True,
+        allow_methods=["*"],
+        allow_headers=["*"],
+    )
+    from app.api.v1.health import router as health_router
+
+    app.include_router(api_router, prefix="/api/v1")
+    app.include_router(health_router)
+    return app
+
+
+app = create_app()