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

frontend/index.html

@@ -627,6 +627,30 @@
                                         >
                                     </span>
                                 </button>
+                                <button
+                                    class="menu-list-row menu-list-button"
+                                    id="api-docs-menu-row"
+                                    type="button"
+                                    role="menuitem"
+                                >
+                                    <span
+                                        class="menu-list-icon"
+                                        aria-hidden="true"
+                                    >
+                                        <i
+                                            data-lucide="code-2"
+                                            style="width: 17px; height: 17px"
+                                        ></i>
+                                    </span>
+                                    <span class="menu-list-content">
+                                        <span class="menu-list-title" data-i18n="header.menu_api"
+                                            >API</span
+                                        >
+                                        <span class="menu-list-subtitle" data-i18n="header.menu_api_desc"
+                                            >Documentation OpenAPI</span
+                                        >
+                                    </span>
+                                </button>
                                 <button
                                     class="menu-list-row menu-list-button"
                                     id="config-open-btn"