chore(01-01): add FERNET_KEY config and email tables migration

- Add FERNET_KEY to .env.example with generation instructions
- Register email.models in aerich_config.py and app.py
- Register email blueprint in app.py
- Create database migration for email_accounts, email_sync_status, emails
- Migration includes proper foreign keys and indexes

aerich_config.py

@@ -16,6 +16,7 @@ TORTOISE_ORM = {
             "models": [
                 "blueprints.conversation.models",
                 "blueprints.users.models",
+                "blueprints.email.models",
                 "aerich.models",
             ],
             "default_connection": "default",