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/stats.py

@@ -7,7 +7,7 @@ from zoneinfo import ZoneInfo
 import aiosqlite
 
 from server.config_loader import load_meds_config, today_str
-from server.db import new_id, utc_now_iso
+from server.db import utc_now_iso
 from server.slots import compute_status, get_log_for_day, get_snooze_for_day