feat(ai): Vercel AI SDK provider factory with BYOK and per-use-case model prefs

Provider factory (OpenAI/Anthropic/OpenRouter/Ollama). generateObject for all outputs.
Features: recipe generation, photo-to-recipe vision, variations, drink/meal pairings,
ingredient substitution, recipe adaptation, nutrition analysis, URL import, meal plan gen.
User BYOK keys (AES-256-GCM). Per-use-case model preferences (text/vision/mealPlan).
Site-level key override via admin settings.

apps/web/lib/ai/features/substitute-ingredient.ts

@@ -0,0 +1,32 @@
+import { generateObject } from "ai";
+import { z } from "zod";
+import { resolveModel, type AiConfig } from "../factory";
+
+const SubstitutionsSchema = z.object({
+  substitutions: z.array(z.object({
+    name: z.string(),
+    ratio: z.string(),
+    note: z.string(),
+    flavorImpact: z.enum(["minimal", "moderate", "significant"]),
+  })),
+});
+
+export type Substitution = z.infer<typeof SubstitutionsSchema>["substitutions"][number];
+
+export async function substituteIngredient(
+  ingredient: string,
+  recipeContext: string,
+  config?: AiConfig
+): Promise<Substitution[]> {
+  const model = resolveModel(config);
+
+  const { object } = await generateObject({
+    model,
+    schema: SubstitutionsSchema,
+    system:
+      "You are a professional chef specializing in ingredient substitutions. Provide practical, commonly available substitutes. For each substitute: name what to use, the ratio (e.g. '1:1', '3/4 the amount'), and a brief note on technique adjustments. Rate the flavor impact honestly.",
+    prompt: `Suggest 3-4 substitutes for "${ingredient}" in this recipe context: ${recipeContext}`,
+  });
+
+  return object.substitutions;
+}