Point correlation at models that still exist, and make a dead one loud

Both Gemini model IDs had been retired by Google. Production logs show every
correlation call 404ing -- "models/gemini-2.0-flash is no longer available" --
and gemini-1.5-pro is gone from the model list as well. Because a failed LLM
call falls back to the rules decision by design, nothing surfaced: the pipeline
kept producing incidents, so the LLM tier and the consensus tiebreak were dead
in production for an unknown number of days while correlation was being tuned.
Some of what recent tuning was reacting to was rules-only behaviour that was
never meant to run alone.

Cheap model becomes gemini-3.6-flash, which is the migration target named in
Google's own 404. Smart model becomes gemini-2.5-pro, the only stable Pro-tier
text model left; the tiebreak fires rarely and its value comes from being a
different, stronger model than the first pass, so a second Flash was not worth
the consensus it would give up. Model list checked against
https://ai.google.dev/gemini-api/docs/models on 2026-08-18.

The more important half is the logging. A per-call WARNING was the only signal,
and it is indistinguishable from an ordinary API hiccup, so a permanent
misconfiguration read as noise. Failures that look like a missing model (404,
"not found", "no longer available") now log once per model at ERROR, name the
config keys to change, and say plainly that correlation is running rules-only.
Transient errors keep the old per-call WARNING. Once per model, not once per
call, so the alert stays readable at radio traffic volume.

Gemini is used nowhere else in c2-core -- extraction, embeddings and summaries
all run on OpenAI -- so the blast radius was exactly the correlation LLM tier.

38 correlator tests still pass. No new environment variables: both model IDs
are config.py defaults and are not templated into any .env, so CI deploys this
without an ansible run.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

drb-c2-core/app/config.py

@@ -38,8 +38,14 @@ class Settings(BaseSettings):
     # Correlation consensus models
     # corr_cheap_model  — first-pass LLM correlator (runs on every call)
     # corr_smart_model  — tiebreaker (only fires when rules and cheap LLM disagree)
-    corr_cheap_model: str = "gemini-2.0-flash"
-    corr_smart_model: str = "gemini-1.5-pro"
+    # Both IDs below were retired by Google and returned 404 on every call from
+    # some point before 2026-08-18 until they were corrected. Because a failed
+    # LLM call falls back to the rules decision, nothing broke loudly -- the
+    # entire LLM tier and the consensus tiebreak were simply dead in production
+    # 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)
     summary_interval_minutes: int = 2          # how often the summary loop runs
     correlation_window_hours: int = 2          # slow/location path: max hours since last call
     embedding_similarity_threshold: float = 0.93  # slow-path: requires location corroboration