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/lib/site-settings.ts

@@ -22,7 +22,8 @@ export type SiteSettingKey =
   | "GITEA_URL"
   | "GITEA_TOKEN"
   | "GITEA_REPO"
-  | "GITEA_WEBHOOK_SECRET";
+  | "GITEA_WEBHOOK_SECRET"
+  | "USDA_API_KEY";
 
 const SECRET_KEYS: SiteSettingKey[] = [
   "OPENAI_API_KEY",
@@ -31,6 +32,7 @@ const SECRET_KEYS: SiteSettingKey[] = [
   "VAPID_PRIVATE_KEY",
   "GITEA_TOKEN",
   "GITEA_WEBHOOK_SECRET",
+  "USDA_API_KEY",
 ];
 
 export function isSecretKey(key: SiteSettingKey): boolean {
@@ -74,6 +76,7 @@ export async function getAllSiteSettings(): Promise<Record<string, { value: stri
     "GITEA_TOKEN",
     "GITEA_REPO",
     "GITEA_WEBHOOK_SECRET",
+    "USDA_API_KEY",
   ];
 
   const result: Record<string, { value: string | null; isSecret: boolean; fromDb: boolean }> = {};