Add multi-token authentication with client labels

- Replace single api_token with api_tokens dict (label: token pairs)
- Add context-aware logging to track which client made each request
- Implement token label lookup with secure comparison
- Add logging middleware to inject token labels into request context
- Update logging format to display [label] in all log messages
- Fix WebSocket authentication to use new multi-token system
- Update CLI --show-token to display all tokens with labels
- Update config generation to use api_tokens format
- Update README with multi-token documentation
- Update config.example.yaml with multiple token examples

Benefits:
- Easy identification of clients in logs (Home Assistant, mobile, web UI, etc.)
- Per-client token management and revocation
- Better security and auditability

Example log output:
2026-02-06 03:36:20,806 - [home_assistant] - WebSocket client connected

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

config.example.yaml

@@ -2,8 +2,12 @@
 # Copy this file to config.yaml and customize as needed.
 # A secure token will be auto-generated on first run if not specified.
 
-# API Token (generate a secure random token)
-api_token: "your-secure-token-here"
+# API Tokens - Multiple tokens with friendly labels
+# This allows you to identify which client is making requests in the logs
+api_tokens:
+  home_assistant: "your-home-assistant-token-here"
+  mobile: "your-mobile-app-token-here"
+  web_ui: "your-web-ui-token-here"
 
 # Server settings
 host: "0.0.0.0"