feat(rust): wire downstream-key billing, add /v1/responses, ComfyUI shim

Previously every executor took `_state: AppState` (unused) and `check_auth`
discarded the matched downstream key id, so `usage_logs` was orphan code.
This change plumbs the key id from auth → handlers → executors → billing,
adds the OpenAI Responses API endpoint, and exposes shim aliases under
`/api/comfyui-agent/proxy/openai/*` so embedded ComfyUI's OpenAI nodes
flow through the same auth/conductor/billing pipeline.

- check_auth now returns Option<i64> (managed key id) instead of unit
- record_billing helper writes usage_logs after each successful upstream call
- extract_usage handles both chat-completions and responses usage shapes
- conductor wrappers record billing post-Success; stream variant scans
  chunks in reverse for the final usage event
- POST /v1/responses (stream + non-stream) via ProxyConductor
- shim aliases mounted on the same Router so auth/failover/billing work for free
- ComfyUIProcess injects BOOSAPI_BUILTIN_KEY env + --comfy-api-base on spawn
- bootstrap built-in `comfyui-agent` downstream key at startup (idempotent)
- get_auth_header in vendored ComfyUI falls back to BOOSAPI_BUILTIN_KEY env

Co-Authored-By: Kiro <noreply@kiro.dev>

src/proxy/executors/mod.rs

@@ -70,9 +70,23 @@ pub async fn execute_chat_completions_stream(
     _state: AppState,
     route: UpstreamRoute,
     body: Value,
+) -> AppResult<Vec<String>> {
+    execute_streaming_request(route, "/chat/completions", body).await
+}
+
+/// Execute a streaming request against any upstream path (e.g. /responses).
+/// Returns SSE `data: ...` lines verbatim.
+pub async fn execute_streaming_request(
+    route: UpstreamRoute,
+    path_suffix: &str,
+    body: Value,
 ) -> AppResult<Vec<String>> {
     let client = http_client();
-    let upstream_url = format!("{}/chat/completions", route.base_url.trim_end_matches('/'));
+    let upstream_url = format!(
+        "{}/{}",
+        route.base_url.trim_end_matches('/'),
+        path_suffix.trim_start_matches('/')
+    );
 
     let resp = client
         .post(&upstream_url)
@@ -94,7 +108,6 @@ pub async fn execute_chat_completions_stream(
         )));
     }
 
-    // Read the streaming response as chunks
     let mut chunks = Vec::new();
     let mut stream = resp.bytes_stream();
     use futures::StreamExt;
@@ -102,7 +115,6 @@ pub async fn execute_chat_completions_stream(
         match chunk_result {
             Ok(bytes) => {
                 let text = String::from_utf8_lossy(&bytes).to_string();
-                // Parse SSE lines from the chunk
                 for line in text.lines() {
                     if line.starts_with("data: ") {
                         chunks.push(line.to_string());