feat: add source_status and degraded fields to HybridRAG response

Each retrieval leg (vector, graph, web, volatile, documents) now returns
(results, timing, error) instead of swallowing exceptions to an empty list.
The response reports per-leg status ('ok'/'failed'/'disabled') in
source_status and sets degraded=true when any enabled leg failed. Failed
legs still contribute no results (behavior unchanged) and are logged at
WARNING. Both fields are additive with defaults, so deploy order relative
to consumers does not matter.

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

src/models/hybrid_rag.py

@@ -88,6 +88,11 @@ class HybridRAGResponse(BaseModel):
     timing: TimingBreakdown = Field(..., description="Performance breakdown")
     config_used: HybridRAGConfig = Field(..., description="Configuration used")
     search_id: Optional[str] = Field(None, description="Search ID for Librarian tracking")
+    source_status: Dict[str, str] = Field(
+        default={},
+        description="Per-leg retrieval status ('ok', 'failed', or 'disabled') keyed by: vector, graph, web, volatile, documents"
+    )
+    degraded: bool = Field(default=False, description="True when any enabled retrieval leg reported 'failed'")
 
 
 class HybridRAGRequest(BaseModel):