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/services/knowledge/__init__.py

@@ -0,0 +1,56 @@
+from __future__ import annotations
+
+from functools import lru_cache
+from typing import Literal, Protocol
+
+from pydantic import BaseModel
+
+from app.core.config import get_settings
+
+EvidenceState = Literal["grounded", "insufficient", "unavailable"]
+
+
+class SourceCard(BaseModel):
+    document_id: str
+    title: str
+    version: str
+    section: str
+    excerpt: str
+
+
+class GroundedAnswer(BaseModel):
+    answer: str
+    evidence_state: EvidenceState
+    sources: list[SourceCard]
+    provider: str
+    correlation_id: str
+
+
+class KnowledgeHealth(BaseModel):
+    provider: str
+    available: bool
+    detail: str
+    tenant: str
+    workspace: str
+    collection: str
+    document_count: int
+
+
+class KnowledgeProvider(Protocol):
+    name: str
+
+    def health(self) -> KnowledgeHealth: ...
+
+    def ask(self, question: str, correlation_id: str) -> GroundedAnswer: ...
+
+
+@lru_cache
+def get_knowledge_provider() -> KnowledgeProvider:
+    settings = get_settings()
+    if settings.knowledge_provider == "ragcore":
+        from app.services.knowledge.ragcore import RAGcoreKnowledgeProvider
+
+        return RAGcoreKnowledgeProvider()
+    from app.services.knowledge.demo import DemoKnowledgeProvider
+
+    return DemoKnowledgeProvider()