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/events.py

@@ -0,0 +1,43 @@
+from typing import Any
+
+from fastapi import APIRouter, Body, Depends, HTTPException
+from pydantic import BaseModel
+from sqlalchemy.orm import Session
+
+from app.auth.api_key import get_api_key_auth
+from app.config import get_settings
+from app.database import get_db
+from app.services.ingest import ingest_event
+from app.services.schema_validate import validate_event_payload
+
+router = APIRouter(prefix="/events", tags=["events"])
+
+
+class IngestResponse(BaseModel):
+    status: str
+    event_id: str
+    created: bool
+    sac_event_url: str
+
+
+@router.post("", status_code=202, response_model=IngestResponse)
+def post_event(
+    payload: dict[str, Any] = Body(...),
+    db: Session = Depends(get_db),
+    _api_key: str = Depends(get_api_key_auth),
+) -> IngestResponse:
+    errors = validate_event_payload(payload)
+    if errors:
+        raise HTTPException(status_code=422, detail={"schema_errors": errors[:20]})
+
+    event, created = ingest_event(db, payload)
+    db.commit()
+
+    settings = get_settings()
+    base = settings.sac_public_url.rstrip("/")
+    return IngestResponse(
+        status="accepted",
+        event_id=event.event_id,
+        created=created,
+        sac_event_url=f"{base}/api/v1/events/{event.id}",
+    )