Explicit timeouts on every OpenAI and Gemini request (#153)

No AI client set a timeout: OpenAI's SDK waits 600s per attempt and retries
twice (timeouts included); google-genai waits forever. A hung provider call
had no upper bound, so recorrelation_sweep's 15-minute pipeline hold-back was
a guess against an unbounded tail.

- config: stt_timeout_seconds (120), openai_timeout_seconds (60),
  openai_max_retries (1), gemini_timeout_seconds (60).
- app/internal/ai_clients.py builds every OpenAI/AsyncOpenAI client with them
  (transcription, extraction, embeddings, summarizer, vocabulary learner,
  trip chat). gemini.py's shared client gets HttpOptions(timeout=ms).
- A timeout raises like any other provider error, so existing fallbacks are
  unchanged: rules-only correlation, rules decision on tiebreak, raw
  transcript on correction, ai_health "transient".
- MIN_MINUTES_SINCE_PIPELINE_START is now derived from those timeouts
  (pipeline_worst_case_seconds, 3-scene budget + 2 min non-AI slack), floor
  15: 21 min with the defaults. Loop warns if it reaches the scan window.
- tests/test_ai_timeouts.py: client limits, a real hung-socket request that
  gives up in <5s, timeout fallback per tier, the derived bound, and a guard
  against any new OpenAI( outside ai_clients.py.

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

drb-c2-core/app/routers/trips.py

@@ -531,8 +531,8 @@ async def trip_chat(
 
     events = await fstore.collection_list("trip_events", trip_id=trip_id)
 
-    from openai import AsyncOpenAI
-    oai = AsyncOpenAI(api_key=settings.openai_api_key)
+    from app.internal.ai_clients import async_openai_client
+    oai = async_openai_client()
 
     # Strip history to only user/assistant roles to prevent prompt injection
     safe_history = [