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

.env.example

@@ -5,6 +5,10 @@ APP_PIN=1234
 JWT_SECRET=change-me-to-a-long-random-string
 JWT_EXPIRE_DAYS=90
 
+# Agent API — zufälliger langer String für externe Agents (openssl rand -hex 32)
+# Leer lassen deaktiviert /agent/* Endpunkte.
+AGENT_TOKEN=
+
 # VAPID keys für Web Push — generieren mit: python tools/gen_vapid.py
 VAPID_PRIVATE_KEY=
 VAPID_PUBLIC_KEY=