feat: regenerate recipe with modifications from the editor

Every existing AI entry point (generate, generate-from-idea, adapt,
variations) either creates a new recipe or a saved variation — none
let you revise the draft you're currently editing in place. Adds a
stateless /api/v1/ai/regenerate endpoint that takes the editor's
current in-progress fields (not a recipeId, so unsaved edits are
included) plus a free-text instruction, and returns a full revised
draft the editor merges into its own state. No DB write happens;
the user still saves normally.

v0.42.0

apps/web/lib/changelog.ts

@@ -1,5 +1,5 @@
 // Mirrors CHANGELOG.md at the repo root — update both together.
-export const APP_VERSION = "0.41.0";
+export const APP_VERSION = "0.42.0";
 
 export type ChangelogEntry = {
   version: string;
@@ -11,6 +11,13 @@ export type ChangelogEntry = {
 };
 
 export const CHANGELOG: ChangelogEntry[] = [
+  {
+    version: "0.42.0",
+    date: "2026-07-17 15:15",
+    added: [
+      "Recipe editor now has \"Regenerate with AI\" — describe what to change (e.g. \"make it spicier\", \"swap in chicken\") and the draft you're editing updates in place, no new recipe created. You still need to save.",
+    ],
+  },
   {
     version: "0.41.0",
     date: "2026-07-17 14:45",