038: add @RATIONALE/@REJECTED contracts to async C4/C5 modules

5 contracts updated:
  - AsyncNetworkModule (C5): async migration rationale, per-client CSRF cookie rejection
  - AsyncAPIClient (C4): auth lifecycle, cache-hit CSRF refresh
  - AsyncAPIClient.request (C4): string vs dict handling, double-encoding root cause
  - AsyncAPIClient.upload_file (C4): multipart async upload
  - LLMAsyncHttpClient (C4): response.ok -> is_success, module-level httpx singleton

backend/src/plugins/translate/_llm_async_http.py

@@ -7,8 +7,11 @@
 # @POST Returns (response text, finish_reason) tuple.
 # @SIDE_EFFECT Async HTTP POST to LLM API with optional retry on 429.
 # @RATIONALE Async migration of _llm_http.py to use httpx.AsyncClient instead of sync requests.
-#            Uses asyncio.sleep for 429 backoff instead of time.sleep.
+#   Uses asyncio.sleep for 429 backoff instead of time.sleep. Module-level httpx client singleton
+#   for connection reuse. response.ok -> response.is_success for httpx.Response compatibility.
 # @REJECTED Keeping sync requests.post — would block async event loop during LLM calls.
+#   Per-request httpx.AsyncClient — loses connection pooling. response.ok — httpx.Response has no
+#   .ok attribute (only is_success), causes 'Response' object has no attribute 'ok'.
 
 import asyncio
 import os