feat(api,ai): #72 OpenAPI 3.1 enrichie + fiabilisation de l'outil AI UI

API (#72):
- backend/openapi_docs.py: 18 tags documentes, assignation auto par prefixe,
  securite bearerAuth/cookieAuth, erreurs 401/403/404/422/500, exemples,
  page /api autonome (liens /docs, /redoc, /openapi.json)
- backend/schemas.py: response_model Pydantic pour ~35 endpoints sans modele
- main.py: FastAPI enrichi + override app.openapi; routes /api et /api/
- ai_routes/bookslm_routes: response_model + doc SSE
- frontend: entree 'API' du menu (i18n FR/EN)
- tests/test_openapi.py (58 tests)

AI UI:
- BooksLM: requetes authentifiees (AuthManager), parsing SSE {token}/error,
  message utilisateur correct (plus le placeholder vide), barre de contexte
- AI Editor: Ctrl/Cmd+J lie une seule fois, libelles i18n, modale de
  reecriture accessible a la place de window.prompt()
- tests/frontend/ai.test.mjs (7 tests) + CI

backend/bookslm_routes.py

@@ -11,6 +11,7 @@ from pydantic import BaseModel, Field
 from backend.auth.middleware import check_vault_access, require_auth
 from backend.bookslm import build_system_prompt, collect_directory_context
 from backend.indexer import get_vault_data
+from backend.schemas import BooksLMContextResponse
 
 logger = logging.getLogger("obsigate.bookslm_routes")
 router = APIRouter(prefix="/api/ai/bookslm", tags=["BooksLM"])
@@ -46,7 +47,7 @@ class BooksLMChatRequest(BaseModel):
 # ── Endpoints ──
 
 
-@router.post("/context")
+@router.post("/context", response_model=BooksLMContextResponse)
 async def api_bookslm_context(
     req: BooksLMContextRequest,
     current_user=Depends(require_auth),
@@ -67,7 +68,11 @@ async def api_bookslm_context(
     return context
 
 
-@router.post("/chat")
+@router.post(
+    "/chat",
+    response_class=StreamingResponse,
+    responses={200: {"content": {"text/event-stream": {}}, "description": "SSE token stream"}},
+)
 async def api_bookslm_chat(
     req: BooksLMChatRequest,
     current_user=Depends(require_auth),