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/settings.py

@@ -10,6 +10,8 @@ class Settings(BaseSettings):
     jwt_secret: str = "dev-secret-change-me"
     jwt_expire_days: int = 90
 
+    agent_token: str = ""  # long-lived token for /agent/* endpoints
+
     vapid_private_key: str = ""
     vapid_public_key: str = ""
     vapid_claims_email: str = "mailto:admin@schwenk.online"
@@ -22,7 +24,8 @@ class Settings(BaseSettings):
     messages_path: str = "messages.yaml"
 
     public_dir: str = "public"
-    app_version: str = "1.1.0"
+    app_version: str = "1.2.0"
+    migrations_dir: str = "tools/migrations"
 
 
 settings = Settings()  # type: ignore[call-arg]