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>

FEATURE_AUDIT.md

@@ -66,7 +66,7 @@ Status legend: **Exists** (fully working) · **Partial** (works but with a real
 | Photo scan (pantry) | Exists | Vision-model based | `apps/web/app/api/v1/pantry/scan/photo` |
 | Nutrition goals | Exists | | `apps/web/app/api/v1/users/me/nutrition-goals` |
 | Nutrition diary | Exists | Single-day diary view, plus a Trend tab (7/30/90-day calorie line chart + daily macro averages) — the same endpoint switches modes via a `range` query param | `apps/web/app/api/v1/users/me/nutrition-diary`, `apps/web/components/nutrition/nutrition-trend.tsx` |
-| **Barcode/USDA nutrition-facts database** | **Missing** | Barcode scan only IDs the product name, doesn't pull nutrition facts; all nutrition numbers are AI-estimated, never database-sourced | — |
+| USDA nutrition-facts database | Exists | `estimateNutrition` now looks up each ingredient in USDA FoodData Central (SR Legacy + Survey (FNDDS) datasets) after converting its quantity/unit to grams, sums the matches, and only falls back to AI estimation for ingredients that couldn't be converted (count-based units like "2 cloves") or had no USDA match — including every ingredient, transparently, when `USDA_API_KEY` isn't configured. Barcode scan (pantry) still only IDs product names via Open Food Facts, unrelated to this. | `apps/web/lib/usda.ts`, `apps/web/lib/ingredient-grams.ts`, `apps/web/lib/ai/features/estimate-nutrition.ts` |
 | Batch cooking | Exists | Shared prep steps grouped by which dish they serve, per-dish fridge/freezer countdown independent of the whole-batch pantry deduction | `apps/web/components/recipe/batch-cook-*.tsx` |
 
 ---
@@ -143,7 +143,7 @@ Ranked roughly by likely value:
 1. **Stripe checkout + billing portal** — the webhook consumer is production-ready but nothing produces the checkout event or lets a user self-serve upgrade/cancel/view invoices. Biggest gap if monetizing seriously.
 2. **Recipe video** — no support at all.
 3. ~~Nutrition trend/history view~~ — closed 2026-07-22 (7/30/90-day calorie chart + macro averages).
-4. **USDA/nutrition-database lookup** — all nutrition numbers are AI-estimated; barcode scan only gets product name, not nutrition facts.
+4. ~~USDA/nutrition-database lookup~~ — closed 2026-07-22 (per-ingredient, gram-convertible ingredients only; AI fills the rest).
 5. ~~OS Share Target~~ — closed 2026-07-21 (Chromium/Android only, no Safari equivalent exists).
 6. ~~Push click-through handling~~ — closed 2026-07-21 (`push`/`notificationclick` listeners added to `sw.js`).
 7. ~~Anonymous public link for meal plans~~ — closed 2026-07-21 (read-only, with a QR code).