Add database explorer tab to settings

[timbornemann]

Summary

• add a dedicated /api/database router that lists tables, returns paginated rows, lets rows be inserted/updated/deleted by rowid, and runs read-only SQL queries
• add a localized DatabaseExplorer component and surface it through a new "Database" tab in the settings navigation
• wire the UI up to the backend endpoints so tables can be inspected, edited as JSON, deleted, and queried directly from the app

Testing

• npm run format
• npm run test

---

Codex Task