fix: variations/adapt-recipe silent errors and no-op duplicates

AI adapt/variations flows had no catch on their fetch calls, so a
network failure surfaced as nothing happening rather than an error
toast. They also unconditionally persisted the AI's output even when
it was identical to the source recipe, and the adapt route never
recorded a recipeVariations row (only plain forks did) or enforced
the per-tier recipe-count limit other create paths already check.

v0.39.0

apps/web/lib/recipe-diff.ts

@@ -0,0 +1,31 @@
+type DiffIngredient = { rawName: string; quantity?: string | number | null; unit?: string | null };
+type DiffStep = { instruction: string };
+
+const norm = (s: string) => s.trim().toLowerCase();
+
+/** True if an AI-adapted/varied recipe is functionally identical to its
+ * source — same servings, same ingredients (name/quantity/unit, in order),
+ * same steps. Used to skip persisting a "variation" that changed nothing,
+ * which otherwise clutters recipe history with no-op duplicates. */
+export function isRecipeUnchanged(
+  original: { baseServings: number; ingredients: DiffIngredient[]; steps: DiffStep[] },
+  candidate: { baseServings: number; ingredients: DiffIngredient[]; steps: DiffStep[] }
+): boolean {
+  if (original.baseServings !== candidate.baseServings) return false;
+  if (original.ingredients.length !== candidate.ingredients.length) return false;
+  if (original.steps.length !== candidate.steps.length) return false;
+
+  for (let i = 0; i < original.ingredients.length; i++) {
+    const o = original.ingredients[i]!;
+    const c = candidate.ingredients[i]!;
+    if (norm(o.rawName) !== norm(c.rawName)) return false;
+    if (String(o.quantity ?? "") !== String(c.quantity ?? "")) return false;
+    if (norm(o.unit ?? "") !== norm(c.unit ?? "")) return false;
+  }
+
+  for (let i = 0; i < original.steps.length; i++) {
+    if (norm(original.steps[i]!.instruction) !== norm(candidate.steps[i]!.instruction)) return false;
+  }
+
+  return true;
+}