AI providers: correlation decision and transcript correction move to OpenAI gpt-4o-mini

One funded account (OpenAI) now covers extraction, embeddings, summaries,
the ambiguous-scene correlation call and transcript correction; Groq stays
optional for STT. New app/internal/llm.py picks the provider from the model
name, so setting either model back to a gemini-* ID needs no code change.
Usage accounting for replay runs now covers both providers.

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

drb-c2-core/app/config.py

@@ -133,12 +133,12 @@ class Settings(BaseSettings):
     # a failed LLM call falls back to the rules decision, so nothing broke
     # loudly. Verify against https://ai.google.dev/gemini-api/docs/models
     # before changing.
-    corr_cheap_model: str = "gemini-3.6-flash"   # was gemini-2.0-flash (shut down)
+    corr_cheap_model: str = "gpt-4o-mini"   # OpenAI since 2026-11-08 (was gemini-3.6-flash); a gemini-* ID still works
     # Transcript correction (server-26#36). Runs inside transcription, once per
     # transcribed call above MIN_WORDS_FOR_CORRECTION, so it is priced like STT
     # rather than like the correlation tier — cheap model on purpose.
     transcript_correction_enabled: bool = True
-    transcript_correction_model: str = "gemini-3.6-flash"
+    transcript_correction_model: str = "gpt-4o-mini"  # OpenAI since 2026-11-08; a gemini-* ID still works
     # Retry Whisper once when its output is degenerate. The same clip produced a
     # 56-word ten-code counting run on one attempt and real speech on the next
     # (2026-08-23, call e49ea32c), so a hallucination is a coin-flip rather than