Fix SDK authentication to return consistent error responses

[maoxiaowang]

Fix SDK authentication to return consistent error responses.

Previously, missing or invalid API keys returned HTTP 200 with inconsistent error messages and codes.
This made it unreliable for SDK clients to detect authentication failures and handle them correctly.

What problem does this PR solve?

• Updated token_required decorator to:
  • Preserve tenant_id injection
  • Raise Unauthorized for missing or invalid API keys
• Added a global app.errorhandler for Unauthorized:
  • Returns structured error response with HTTP 401
• Authentication failures now consistently return either:
  • HTTP 401 or response code 401 (for legacy compatibility)

Frontend Impact

• Frontend now properly handles HTTP 401 responses, and redirects to login page as expected.
• No changes to existing pages or authentication logic.

Type of change

• Performance Improvement