feat: overhaul with modular API, agent endpoints, migrations, backup and tests

- Split monolithic app.py into server/api/ routers
- Add Pydantic schemas for input validation
- Add /agent/* API with separate AGENT_TOKEN auth
- Replace static schema.sql with versioned migrations (datenerhaltend)
- Add tools/backup_db.py for SQLite backups
- Add Dockerfile HEALTHCHECK and compose.yml AGENT_TOKEN env
- Add pytest tests for user and agent API
- Update README with API docs and deployment safety notes
- Bump version to 1.2.0

server/api/health.py

@@ -0,0 +1,18 @@
+from __future__ import annotations
+
+from fastapi import APIRouter
+from fastapi.responses import JSONResponse
+
+from server.settings import settings
+
+router = APIRouter(prefix="/api/health", tags=["health"])
+
+NO_STORE = {"Cache-Control": "no-store"}
+
+
+@router.get("/")
+async def health_check() -> JSONResponse:
+    return JSONResponse(
+        {"status": "ok", "version": settings.app_version},
+        headers=NO_STORE,
+    )