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/main.rs

@@ -1,5 +1,6 @@
+use anyhow::Context;
 use axum::serve;
-use boosapi::{config::AppConfig, db, AppState};
+use boosapi::{config::AppConfig, db, services::builtin_keys, AppState};
 use clap::Parser;
 use std::ffi::OsString;
 use std::net::SocketAddr;
@@ -85,8 +86,20 @@ async fn main() -> anyhow::Result<()> {
         return Ok(());
     }
 
+    // Bootstrap built-in system keys (idempotent).
+    let comfyui_agent_key = builtin_keys::ensure_comfyui_agent_key(&pool)
+        .await
+        .context("failed to bootstrap built-in ComfyUI Agent key")?;
+    tracing::info!(
+        key_id = comfyui_agent_key.0,
+        "Built-in ComfyUI Agent downstream key ready"
+    );
+    let builtin = builtin_keys::BuiltinKeys {
+        comfyui_agent: comfyui_agent_key,
+    };
+
     // Build app state
-    let state = AppState::new(config.clone(), pool);
+    let state = AppState::new(config.clone(), pool, builtin);
     if let Err(err) = state.comfyui.start().await {
         tracing::warn!(%err, "ComfyUI embedded process did not start");
     }