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/messages/en.json

@@ -111,6 +111,16 @@
     "adaptButton": "Adapt recipe",
     "clearExclusions": "Clear",
     "variationsDirectionsPlaceholder": "e.g. make it vegan, use only pantry staples, reduce sugar…",
+    "regenerateWithAi": "Regenerate with AI",
+    "regenerateTitle": "Regenerate with AI",
+    "regenerateDescription": "Describe what to change — the rest of the recipe stays as-is. This updates the draft you're editing; you'll still need to save.",
+    "regenerateInstructionLabel": "What should change?",
+    "regenerateInstructionPlaceholder": "e.g. make it spicier, swap in chicken instead of beef, halve the servings…",
+    "regenerateCancel": "Cancel",
+    "regenerating": "Regenerating…",
+    "regenerateApply": "Regenerate",
+    "regenerated": "Draft updated — review and save your changes.",
+    "regenerateFailed": "Failed to regenerate recipe",
     "historyRestoreFailed": "Failed to restore version",
     "pairingMealFailed": "Failed to suggest pairings",
     "pairingDrinkFailed": "Failed to suggest drinks",