feat(rag): stream grounded RAG answers over SSE with source citations

Phase 03 (Story: Chat RAG Answer — happy path):
- app/rag/retriever.py: top-k cosine search + parent-doc selection with
  per-doc dedupe and BOR_MAX_CONTEXT_CHARS cap ([…truncated…] marker)
- app/rag/prompts.py: locked persona + HIGH/DEFLECT prompt builders
- app/rag/llm.py: LLMError + chat_stream (turbo, temp 0.4, max 700, stream)
- app/api/chat.py: POST /api/chat SSE — delta* then done{deflected,
  sources, suggestions}; query_log row + PLAN §9 per-turn log line;
  structured error event on mid-stream failure, JSON 503 when DB down
- frontend: SSE reader, live bubble streaming, source chips -> /sources.html,
  red role=alert banner, Send button state that always recovers
- fix(scaffold): [hidden] { display: none !important } — .kb-banner's
  display:flex was overriding the hidden attribute (banner always visible)
- tests: unit (retriever/prompts/sse/llm) + integration (real Postgres RAG
  turn, query_log, error + 503 paths, mid-turn failures) + Playwright story
  suite (grounded answer, log row, raw SSE shape); smoke placeholder test
  replaced with the real never-stale-button contract

tests/integration/test_api.py

@@ -34,16 +34,6 @@ def test_styles_and_js_served(client) -> None:
     assert client.get("/assets/app.js").status_code == 200
 
 
-def test_chat_placeholder_roundtrip(client) -> None:
-    r = client.post("/api/chat", json={"message": "hello brain"})
-    assert r.status_code == 200
-    data = r.json()
-    assert data["ok"] is True
-    assert "neurons" in data["answer"]
-    assert data["deflected"] is False
-    assert data["sources"] == []
-
-
 def test_chat_requires_message(client) -> None:
     r = client.post("/api/chat", json={"message": ""})
     assert r.status_code == 422