fix(pantry): make public ingredient endpoint idempotent

Backend:
- POST /api/ingredients now checks name_lower and aliases before inserting
- Returns existing ingredient on 409 instead of throwing error

Frontend:
- Removed fragile 409-recovery logic from Pantry.tsx handleAdd
- Added aliases field to Ingredient type for case-insensitive matching

Fixes pantry add for ingredients like 'Carrots' whose canonical name is 'Carrot'

docs/ARCHITECTURE.md

@@ -299,6 +299,7 @@ Generate plan → Send proposal email (per-member tokens)
 | POST | `/api/pantry` | Add item to pantry |
 | DELETE | `/api/pantry/{id}` | Remove item from pantry |
 | POST | `/api/feedback` | Submit meal feedback |
+| POST | `/api/ingredients` | Create or resolve ingredient (public, idempotent) |
 | GET | `/api/recipes` | Search recipes |
 | POST | `/api/admin/scrape` | Trigger grocery scrape (admin) |
 | GET | `/api/admin/logs` | Get scraper logs |