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/models/event.py

@@ -0,0 +1,30 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, String, Text, UniqueConstraint, func
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.database import Base
+
+
+class Event(Base):
+    __tablename__ = "events"
+    __table_args__ = (UniqueConstraint("event_id", name="uq_events_event_id"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
+    event_id: Mapped[str] = mapped_column(String(36), index=True)
+    host_id: Mapped[int] = mapped_column(ForeignKey("hosts.id"), index=True)
+    occurred_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), index=True)
+    received_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    category: Mapped[str] = mapped_column(String(64), index=True)
+    type: Mapped[str] = mapped_column(String(128), index=True)
+    severity: Mapped[str] = mapped_column(String(16), index=True)
+    title: Mapped[str] = mapped_column(String(256))
+    summary: Mapped[str] = mapped_column(Text)
+    details: Mapped[dict | None] = mapped_column(JSONB)
+    raw: Mapped[dict | None] = mapped_column(JSONB)
+    dedup_key: Mapped[str | None] = mapped_column(String(512), index=True)
+    correlation_id: Mapped[str | None] = mapped_column(String(36))
+    payload: Mapped[dict] = mapped_column(JSONB)
+
+    host: Mapped["Host"] = relationship(back_populates="events")