feat: record reranker call outcome into ModelUsageTracker

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/infrastructure/vectorstore/cross_encoder_reranker.py

@@ -9,6 +9,7 @@ from loguru import logger
 
 from app.config.settings import settings
 from app.domain.retrieval import Reranker, RetrievedChunk
+from app.shared.model_usage_tracker import get_model_usage_tracker
 
 
 class OpenAICompatibleReranker(Reranker):
@@ -37,10 +38,26 @@ class OpenAICompatibleReranker(Reranker):
             scores = self._call_reranker(query, texts)
         except Exception as exc:
             logger.warning("Reranker call failed ({}), falling back to original order: {}", type(exc).__name__, exc)
+            # Record the failure so the Status page reflects real reranker health.
+            get_model_usage_tracker().record(
+                provider="reranker",
+                model=self._model,
+                success=False,
+                latency_ms=int((time.time() - start) * 1000),
+                error=str(exc),
+            )
             return chunks[:top_k]
 
         elapsed_ms = int((time.time() - start) * 1000)
         logger.debug("Reranker scored {} chunks in {}ms", len(chunks), elapsed_ms)
+        # TEI/Cohere-style rerank responses carry no token usage field —
+        # only call success/latency is meaningful for this role.
+        get_model_usage_tracker().record(
+            provider="reranker",
+            model=self._model,
+            success=True,
+            latency_ms=elapsed_ms,
+        )
 
         ranked = sorted(
             [(score, chunk) for score, chunk in zip(scores, chunks)],