feat: add API key authentication for AI proxy endpoints

- Add API_KEY config (env var) to all entry points
- requireAuth helper checks Authorization: Bearer or x-api-key header
- Auth applied to /v1/chat/completions, /v1/messages, /v1/models
- When API_KEY is empty/unset, auth is disabled (backward compatible)
- Update wrangler.toml with API_KEY variable documentation

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

wrangler.toml

@@ -12,3 +12,4 @@ workers_dev = true
 # RELAY_TIMEOUT_MS = "30000"
 # RATE_LIMIT_MAX = "200"
 # RATE_LIMIT_WINDOW_MS = "60000"
+# API_KEY = "sk-your-secret-key"