refactor: implement auth middleware for Gin

Replace the old checkAuth() method with a Gin middleware function that validates
Basic Auth credentials. The authMiddleware() now handles authentication at the
middleware level rather than per-handler, supporting graceful degradation when
no auth file is configured.

The middleware:
- Skips auth if AuthFile is empty or doesn't exist
- Extracts Basic Auth credentials from the request
- Verifies credentials using the auth package
- Returns 401 with WWW-Authenticate header on failure
- Calls c.Next() to pass control to handlers on success

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

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