M5: implement RAGcore knowledge integration

KnowledgeProvider protocol with a deterministic TF-IDF-weighted extractive demo provider (never generative, always cites real excerpts) and a RAGcore HTTP adapter that degrades cleanly to unavailable. Knowledge nav + chat-style Q&A UI with source cards and honest grounded/insufficient/unavailable states. 57 backend tests passing, ruff clean. Fixed a real relevance bug (generic terms like "vehicle" crowding out distinctive matches) via IDF weighting, found by testing the actual S6 scenario. Verified end-to-end in the browser: grounded damage question cites both expected procedures; unrelated question honestly returns insufficient evidence with no fabrication.

backend/app/core/config.py

@@ -14,6 +14,8 @@ class Settings(BaseSettings):
     ragcore_tenant: str = "northstar-mobility-demo"
     ragcore_workspace: str = "mobilityops"
     ragcore_collection: str = "internal-procedures"
+    ragcore_api_token: str = ""
+    ragcore_http_timeout_seconds: float = 5.0
     n8n_webhook_url: str = "http://n8n:5678/webhook/mobilityops-return"
     n8n_callback_token: str = "replace-me-n8n-callback-token"
     n8n_dispatch_enabled: bool = True
@@ -24,6 +26,7 @@ class Settings(BaseSettings):
     session_cookie_name: str = "mobilityops_session"
     session_ttl_seconds: int = 60 * 60 * 8
     seed_dir: str = "/app/seed"
+    knowledge_dir: str = "/app/knowledge/procedures"
     cors_allow_origins: str = "http://localhost:1228"
     demo_today: str = "2026-08-01"