correlation: smart tiebreak model gemini-2.5-pro is gone; use gemini-3.8-flash

The first replay (server-26#170) aborted on 5x 'model is unavailable'
from the tiebreak tier. Google's model list shows 2.5 closed to new
projects and no stable Pro model; 3.8-flash is the newest stable.

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

drb-c2-core/app/config.py

@@ -45,7 +45,10 @@ class Settings(BaseSettings):
     # while correlation behaviour was being tuned against rules-only output.
     # 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_smart_model: str = "gemini-2.5-pro"     # was gemini-1.5-pro (shut down)
+    # gemini-2.5-pro was closed to new projects by 2026-09 (every tiebreak 404'd
+    # in the first replay run, server-26#170); Google lists no stable Pro model,
+    # so the smart tier is the newest stable Flash instead.
+    corr_smart_model: str = "gemini-3.8-flash"   # was gemini-2.5-pro, gemini-1.5-pro
     # 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.