feat: persist AI chat history, add search across it

Both chat panels (per-recipe and the general cooking assistant) previously
kept messages in component state only — closing the sheet or reloading
the page lost everything. Both POST routes now persist question+answer
pairs to a new chat_messages table (recipeId null for the general
assistant), and both panels load their own history back on open.

New GET /api/v1/ai/chat-history (scoped by recipeId, or scope=general for
the homepage assistant, or neither to search across everything) backs a
new search control in both panels' header — debounced, shows matched
messages with date and (when searching across everything) which recipe
they belonged to.

Verified locally: asked a real question, closed and reopened the panel
and confirmed the conversation reloaded, then searched a keyword from
that conversation and confirmed both the question and answer surfaced
in the results dropdown.

apps/web/messages/fr.json

@@ -388,6 +388,13 @@
     "inputPlaceholder": "Posez une question…",
     "send": "Envoyer le message"
   },
+  "chatHistory": {
+    "searchAriaLabel": "Rechercher dans l'historique",
+    "searchPlaceholder": "Rechercher une question passée…",
+    "searching": "Recherche…",
+    "noResults": "Aucun message correspondant",
+    "close": "Fermer"
+  },
   "cookingChat": {
     "sorry": "Désolé, je n'ai pas pu répondre à cela.",
     "error": "Une erreur s'est produite. Veuillez réessayer.",
@@ -1153,6 +1160,9 @@
     "lastUsedOn": "Utilisée le {date}",
     "neverUsed": "Jamais utilisée",
     "revoke": "Révoquer",
+    "apiKeyScopeFull": "Accès complet",
+    "apiKeyScopeRead": "Lecture seule",
+    "apiKeyScopeHelp": "Les clés en lecture seule ne peuvent faire que des requêtes GET — elles ne peuvent rien créer, modifier ou supprimer.",
     "copyFailed": "Échec de la copie dans le presse-papiers",
     "byokSaved": "Clé {provider} enregistrée",
     "byokSaveFailed": "Échec de l'enregistrement de la clé",