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/host.py

@@ -0,0 +1,28 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, String, Text, func
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.database import Base
+
+
+class Host(Base):
+    __tablename__ = "hosts"
+
+    id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
+    agent_instance_id: Mapped[str | None] = mapped_column(String(128), unique=True, index=True)
+    hostname: Mapped[str] = mapped_column(String(255), index=True)
+    display_name: Mapped[str | None] = mapped_column(String(255))
+    os_family: Mapped[str] = mapped_column(String(32))
+    os_version: Mapped[str | None] = mapped_column(String(128))
+    product: Mapped[str] = mapped_column(String(64), index=True)
+    product_version: Mapped[str | None] = mapped_column(String(64))
+    ipv4: Mapped[str | None] = mapped_column(String(45))
+    ipv6: Mapped[str | None] = mapped_column(String(45))
+    tags: Mapped[list | None] = mapped_column(JSONB, default=list)
+    use_sac_mode: Mapped[str | None] = mapped_column(String(32))
+    last_seen_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+
+    events: Mapped[list["Event"]] = relationship(back_populates="host")