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/locales/en.json

@@ -87,6 +87,7 @@
   "ai.error_invalid_key": "Invalid API key",
   "ai.explain": "Explain",
   "ai.fix_spelling": "Fix spelling",
+  "ai.fix_spelling_hint": "Fixes spelling and grammar",
   "ai.frontmatter": "Generate frontmatter",
   "ai.generate": "Generate",
   "ai.hint_add_details": "Adds details",
@@ -113,16 +114,20 @@
   "ai.rewrite": "💬 Rewrite",
   "ai.rewrite_done": "AI: text rewritten",
   "ai.rewrite_prompt": "Rewrite instruction:",
+  "ai.rewrite_placeholder": "e.g. make the tone more formal and add a conclusion",
   "ai.rewrite_title": "Rewrite text according to your instructions",
   "ai.select_text": "AI: select text to process",
   "ai.shorter": "Shorter",
+  "ai.shorter_hint": "Makes the text more concise",
   "ai.simplify": "Simplify",
+  "ai.simplify_hint": "Simplifies the language",
   "ai.summarize": "Summarize",
   "ai.text_processed": "AI: text processed ✓",
   "ai.text_rewritten": "AI: text rewritten",
   "ai.to_canvas": "Convert to Canvas",
   "ai.to_list": "Convert to list",
   "ai.to_table": "Convert to table",
+  "ai.tone_hint": "Changes the text tone",
   "ai.toolbar_edit": "Edit",
   "ai.toolbar_generate": "Generate",
   "ai.toolbar_rewrite": "Rewrite",
@@ -130,6 +135,7 @@
   "ai.toolbar_toolbox": "Toolbox",
   "ai.toolbar_translate": "Translate",
   "ai.toolbox": "🧰 Toolbox",
+  "ai.toolbox_hint": "Conversion tools",
   "ai.translate_to": "Translate to {lang}",
   "auth.cannot_delete_self": "Cannot delete your own account",
   "auth.logged_out": "Signed out",
@@ -624,6 +630,8 @@
   "header.menu_about_desc": "Version, credits & info",
   "header.menu_admin": "Administration",
   "header.menu_admin_desc": "Manage users",
+  "header.menu_api": "API",
+  "header.menu_api_desc": "OpenAPI documentation",
   "header.menu_config": "Settings",
   "header.menu_config_desc": "Application settings",
   "header.menu_help": "User guide",
@@ -1597,6 +1605,7 @@
   "bookslm.export": "Export conversation",
   "bookslm.toggle_sidebar": "Hide/Show AI sidebar",
   "bookslm.copy": "Copy",
+  "bookslm.error": "AI service error",
   "bookslm.regenerate": "Regenerate",
   "bookslm.suggestion_summary": "Summarize this directory",
   "bookslm.suggestion_themes": "What are the main themes?",

frontend/locales/fr.json

@@ -87,6 +87,7 @@
   "ai.error_invalid_key": "clé API invalide",
   "ai.explain": "Expliquer",
   "ai.fix_spelling": "Corriger l'orthographe",
+  "ai.fix_spelling_hint": "Corrige les fautes",
   "ai.frontmatter": "Générer frontmatter",
   "ai.generate": "Générer",
   "ai.hint_add_details": "Ajoute des détails",
@@ -113,16 +114,20 @@
   "ai.rewrite": "💬 Réécrire",
   "ai.rewrite_done": "AI: texte réécrit",
   "ai.rewrite_prompt": "Instruction de réécriture :",
+  "ai.rewrite_placeholder": "Ex. : rends le ton plus formel et ajoute une conclusion",
   "ai.rewrite_title": "Réécrit le texte selon vos instructions",
   "ai.select_text": "AI: sélectionnez du texte à traiter",
   "ai.shorter": "Raccourcir",
+  "ai.shorter_hint": "Rend le texte plus concis",
   "ai.simplify": "Simplifier",
+  "ai.simplify_hint": "Simplifie le langage",
   "ai.summarize": "Résumer",
   "ai.text_processed": "AI: texte traité ✓",
   "ai.text_rewritten": "AI: texte réécrit",
   "ai.to_canvas": "Convertir en Canvas",
   "ai.to_list": "Convertir en liste",
   "ai.to_table": "Convertir en tableau",
+  "ai.tone_hint": "Change le ton du texte",
   "ai.toolbar_edit": "Éditer",
   "ai.toolbar_generate": "Générer",
   "ai.toolbar_rewrite": "Réécrire",
@@ -130,6 +135,7 @@
   "ai.toolbar_toolbox": "Toolbox",
   "ai.toolbar_translate": "Traduire",
   "ai.toolbox": "🧰 Boîte",
+  "ai.toolbox_hint": "Outils de conversion",
   "ai.translate_to": "Traduire en {lang}",
   "auth.cannot_delete_self": "Impossible de supprimer son propre compte",
   "auth.logged_out": "Déconnecté",
@@ -624,6 +630,8 @@
   "header.menu_about_desc": "Version, crédits & info",
   "header.menu_admin": "Administration",
   "header.menu_admin_desc": "Gérer les utilisateurs",
+  "header.menu_api": "API",
+  "header.menu_api_desc": "Documentation OpenAPI",
   "header.menu_config": "Configurations",
   "header.menu_config_desc": "Paramètres de l'application",
   "header.menu_help": "Guide d'utilisation",
@@ -1597,6 +1605,7 @@
   "bookslm.export": "Exporter la conversation",
   "bookslm.toggle_sidebar": "Masquer/Afficher la sidebar AI",
   "bookslm.copy": "Copier",
+  "bookslm.error": "Erreur du service AI",
   "bookslm.regenerate": "Régénérer",
   "bookslm.suggestion_summary": "Résume ce répertoire",
   "bookslm.suggestion_themes": "Quels sont les thèmes principaux ?",