security: Add author whitelist for task router (Issue #132)

Implements security fix for issue #132 - Task router author whitelist

Changes:
- Add author_whitelist.py module with whitelist validation
- Integrate whitelist checks into task_router_daemon.py
- Add author_whitelist config option to config.yaml
- Add comprehensive tests for whitelist validation

Security features:
- Validates task authors against authorized whitelist
- Logs all authorization attempts (success and failure)
- Secure by default: empty whitelist denies all
- Configurable via environment variable or config file
- Prevents unauthorized command execution from untrusted Gitea users

config.yaml

@@ -160,6 +160,11 @@ security:
     enabled: false
     domains: []
     shared_files: []
+  # Author whitelist for task router (Issue #132)
+  # Only users in this list can submit tasks via Gitea issues
+  # Empty list = deny all (secure by default)
+  # Set via env var TIMMY_AUTHOR_WHITELIST as comma-separated list
+  author_whitelist: []
 _config_version: 9
 session_reset:
   mode: none