Add application logging, SMTP email notifications, and supporting changes

Phase 5.4 — Application Logging:
- AppLog model + migration 0010 for unified audit/usage/error logging
- Usage logging middleware in app.py (after_request for /api/* requests)
- Error logging in 500 handler with traceback capture
- Audit logging for auth events (register, login, login_failed, logout,
  password_change) and admin actions (backup, restore, user_delete,
  registration_toggle, smtp_config, smtp_test)
- Admin log viewer (LogsView.vue) with stats, category/search/date
  filters, paginated table with expandable detail rows
- Admin logs API endpoints in admin.py (GET /logs, GET /logs/stats)
- Configurable retention via LOG_RETENTION_DAYS with hourly cleanup

Phase 5.5 — SMTP Email Notifications:
- aiosmtplib dependency for async email sending
- Email service (services/email.py) with STARTTLS/implicit TLS support
- Notification service (services/notifications.py) for security alerts
  and task due date reminders with per-user preferences
- Admin SMTP config endpoints (GET/PUT /api/admin/smtp, POST test)
- SMTP config in Config class with env var + Docker secret support
- Settings UI: notification preferences for all users, SMTP config
  section for admin with test email

Other changes:
- stream_chat() now accepts optional options dict (for num_predict)
- Increase assist MAX_BODY_CHARS from 3000 to 8000
- get_user_by_username() added to auth service
- apiStreamPost buffer processing refactored for robustness
- AppHeader: admin Logs nav link
- Router: /admin/logs route

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/fabledassistant/services/auth.py

@@ -87,6 +87,14 @@ async def get_user_by_id(user_id: int) -> User | None:
         return await session.get(User, user_id)
 
 
+async def get_user_by_username(username: str) -> User | None:
+    async with async_session() as session:
+        result = await session.execute(
+            select(User).where(User.username == username)
+        )
+        return result.scalars().first()
+
+
 async def change_password(user_id: int, current_password: str, new_password: str) -> bool:
     """Change a user's password. Returns True on success, False if current password is wrong."""
     async with async_session() as session: