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 messages and codes, making it impossible for SDK clients to detect authentication failures reliably.

What problem does this PR solve?

• Optimize token_required decorator to preserve tenant_id injection.
• token_required now raises Unauthorized exception for missing or invalid API keys.
• Handle Unauthorized exceptions via app.errorhandler, returning code and HTTP 401.
• Frontend can handle both HTTP 401 or code 401 responses and redirect to the login page.

Frontend Impact

• Frontend now properly handles HTTP 401 responses.
• No changes to existing pages or authentication logic.

Type of change

• Performance Improvement