fix: valid key header bypasses both IP allowlist and PROXY_SECRET check

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -242,7 +242,7 @@ docker inspect --format='{{.State.Health.Status}}' api-proxy
 ## How It Works
 
 1. A k8s pod sends a request to `https://proxy.bildme.ru/elevenlabs/v1/text-to-speech/...` (or `http://<vm-ip>:8082/...`) with the `X-Proxy-Token` header and the original API key.
-2. Caddy lets the request through if the source IP is in `ALLOWED_CIDR` **or** it carries the `AUTH_HEADER_NAME: AUTH_HEADER_VALUE` header, then validates `X-Proxy-Token` (if `PROXY_SECRET` is set) — rejecting with 403 on failure. `/health` is answered before both guards.
+2. Caddy lets the request through if the source IP is in `ALLOWED_CIDR` **or** it carries the `AUTH_HEADER_NAME: AUTH_HEADER_VALUE` header, then validates `X-Proxy-Token` (if `PROXY_SECRET` is set; a valid key header satisfies this check too) — rejecting with 403 on failure. `/health` is answered before both guards.
 3. `handle_path` strips the prefix (`/elevenlabs/`, `/openai/`, `/telegram/`, `/openrouter/`, `/deepseek/`) and the request is forwarded over HTTPS with the upstream host as both `Host` header and TLS SNI.
 4. `X-Proxy-Token`, the `AUTH_HEADER_NAME` header and all IP-leaking headers (`X-Forwarded-*`, `X-Real-IP`, `True-Client-IP`, `CF-Connecting-IP`, `X-Client-IP`, `Forwarded`, `Via`) are removed before forwarding; API keys pass through unchanged.
 5. Responses stream back unbuffered (`flush_interval -1`) — critical for binary audio, SSE token streams and Telegram long-polling.