feat: Phase 2A — product cost tracking

- Products: add cost_simple (flat €) and cost_breakdown (JSON line items)
- OrderItems: add unit_cost snapshot written at time of order creation
- Snapshot logic: breakdown sum takes priority over cost_simple; NULL if neither set
- Product schema: serialize/deserialize cost_breakdown as JSON; expose in API
- Product performance report: add trackable_profit, uncosted_revenue, has_gap per product
- Manager UI: cost section in product edit form (simple / detailed toggle with live margin %)
- Products list: show margin % or cost-error badge per product
- Product performance report: profit column + gap warning banner

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/main.py

@@ -262,6 +262,10 @@ def _run_migrations():
             created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
             updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
         )""",
+        # Phase 2A — product cost tracking
+        "ALTER TABLE products ADD COLUMN cost_simple REAL",
+        "ALTER TABLE products ADD COLUMN cost_breakdown TEXT",
+        "ALTER TABLE order_items ADD COLUMN unit_cost REAL",
     ]
     for sql in migrations:
         try: