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/en.json

@@ -388,6 +388,13 @@
     "inputPlaceholder": "Ask a question…",
     "send": "Send message"
   },
+  "chatHistory": {
+    "searchAriaLabel": "Search chat history",
+    "searchPlaceholder": "Search past questions…",
+    "searching": "Searching…",
+    "noResults": "No matching messages",
+    "close": "Close"
+  },
   "cookingChat": {
     "sorry": "Sorry, I couldn't answer that.",
     "error": "Something went wrong. Please try again.",
@@ -1147,6 +1154,9 @@
     "lastUsedOn": "Last used {date}",
     "neverUsed": "Never used",
     "revoke": "Revoke",
+    "apiKeyScopeFull": "Full access",
+    "apiKeyScopeRead": "Read-only",
+    "apiKeyScopeHelp": "Read-only keys can only make GET requests — they can't create, edit, or delete anything.",
     "webhookUrlPlaceholder": "https://example.com/webhook",
     "webhookAdding": "Adding…",
     "webhookAddButton": "Add webhook",