Add consensus correlator: rules + Gemini LLM with smart tiebreaker

Refactor incident_correlator.py to a decision/commit split (preview_correlation
/ apply_correlation) so the rules engine and LLM can both produce decisions before
anything is written to Firestore.

Add llm_correlator.py: cheap Gemini Flash first-pass + Gemini Pro tiebreaker.
Wire _correlate_with_consensus in upload.py — rules-only fallback when key is
absent or call is thin; agreed/tiebreak consensus written to corr_debug.

drb-c2-core/app/config.py

@@ -26,6 +26,11 @@ class Settings(BaseSettings):
 
     # Gemini (intelligence extraction, embeddings, incident summaries)
     gemini_api_key: Optional[str] = None
+    # 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"
     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