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

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 All notable changes to Epicure are documented here. This file is mirrored in-app at `/changelog` (and in the admin dashboard) via `apps/web/lib/changelog.ts` — update both together.
 
+## 0.42.0 — 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.
+
 ## 0.41.0 — 2026-07-17 14:45
 
 ### Added