feat: USDA FoodData Central per-ingredient nutrition lookup (v0.70.0)

estimateNutrition() (lib/ai/features/estimate-nutrition.ts) is now a
hybrid: for each ingredient, estimateGrams() (lib/ingredient-grams.ts)
converts its quantity+unit to grams where possible (weight units
exactly; volume units -- cup/tbsp/tsp/ml/l/etc -- via a 1ml~=1g water-
density approximation, documented as a real simplification but far
better than skipping them). Convertible ingredients get looked up in
USDA FoodData Central (lib/usda.ts, SR Legacy + Survey (FNDDS)
datasets -- the two suited to generic/raw ingredients, not Branded
packaged products or narrower Foundation) and summed. Whatever's left
(count-based units like "2 cloves", no USDA match, or USDA_API_KEY
unset entirely) goes through one AI call asking for just that
subset's total contribution, which sums directly with the USDA
totals -- no whole-recipe AI estimate to reconcile against.

Same exported signature and return shape as before
(NutritionEstimate / {perServing: {...}}), so every existing caller
(the nutrition route, meal-plan generation) gets more accurate
results automatically once USDA_API_KEY is set in Admin -> Settings,
with zero behavior change when it isn't.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/admin/settings/page.tsx

@@ -23,6 +23,14 @@ const SETTING_GROUPS = [
       "never touches code or repo settings, just creates issues.",
     keys: ["GITEA_URL", "GITEA_TOKEN", "GITEA_REPO"] as const,
   },
+  {
+    title: "USDA FoodData Central",
+    description:
+      "Improves recipe nutrition estimates with real per-ingredient data instead of AI-only guesses, " +
+      "for ingredients whose quantity/unit can be converted to grams. Free — get a key at " +
+      "fdc.nal.usda.gov/api-key-signup. Unset falls back to the previous AI-only estimation for every ingredient.",
+    keys: ["USDA_API_KEY"] as const,
+  },
 ];
 
 export default async function AdminSettingsPage() {

apps/web/app/api/v1/admin/settings/route.ts

@@ -26,6 +26,7 @@ const ALLOWED_KEYS: SiteSettingKey[] = [
   "GITEA_TOKEN",
   "GITEA_REPO",
   "GITEA_WEBHOOK_SECRET",
+  "USDA_API_KEY",
 ];
 
 export async function PUT(req: NextRequest) {