feat: clear chat history manually, auto-expire old history after 90 days

Manual: DELETE /api/v1/ai/chat-history (scoped by recipeId or scope=general,
matching GET's scoping) plus a trash-icon button + confirm dialog in both
chat panels' headers.

Automatic: new internal cron endpoint (chat-cleanup), same shared-secret
pattern as the existing leftover-reminders/weekly-digest crons, deletes
any chat_messages older than 90 days. Wired into cron/crontab (daily,
03:00 UTC) and the Dockerfile's cron stage.

Verified locally: cleared a real conversation through the actual UI and
confirmed it didn't come back on reopen (not just cleared client-side);
inserted a 100-day-old and a 5-day-old message directly, called the cron
endpoint with the real shared-secret check, confirmed only the old one
was deleted and the recent one survived; confirmed the endpoint 401s with
no/wrong secret.

apps/web/messages/en.json

@@ -393,7 +393,14 @@
     "searchPlaceholder": "Search past questions…",
     "searching": "Searching…",
     "noResults": "No matching messages",
-    "close": "Close"
+    "close": "Close",
+    "clearAriaLabel": "Clear chat history",
+    "clearConfirmTitle": "Clear this conversation?",
+    "clearConfirmDescription": "This deletes the saved history for this chat. This can't be undone.",
+    "clearCancel": "Cancel",
+    "clearConfirm": "Clear history",
+    "cleared": "Chat history cleared",
+    "clearFailed": "Failed to clear chat history"
   },
   "cookingChat": {
     "sorry": "Sorry, I couldn't answer that.",

apps/web/messages/fr.json

@@ -393,7 +393,14 @@
     "searchPlaceholder": "Rechercher une question passée…",
     "searching": "Recherche…",
     "noResults": "Aucun message correspondant",
-    "close": "Fermer"
+    "close": "Fermer",
+    "clearAriaLabel": "Effacer l'historique",
+    "clearConfirmTitle": "Effacer cette conversation ?",
+    "clearConfirmDescription": "Cela supprime l'historique enregistré de cette conversation. Cette action est irréversible.",
+    "clearCancel": "Annuler",
+    "clearConfirm": "Effacer l'historique",
+    "cleared": "Historique effacé",
+    "clearFailed": "Échec de l'effacement de l'historique"
   },
   "cookingChat": {
     "sorry": "Désolé, je n'ai pas pu répondre à cela.",