feat: Phase 2H — void/waste log

Backend:
- New model: WasteLog (waste_log table) — product, quantity (float), reason, reason_notes,
  unit_cost_snapshot (copied at log time from product's effective cost), total_cost (stored),
  logged_by, business_day_id
- Cost snapshot: same logic as Phase 2A — breakdown sum first, fallback to cost_simple, null if neither
- New router /api/waste/:
  - GET / — list entries (?business_day_id= or ?from=&to=), ordered by logged_at desc
  - POST / — log waste; snapshots cost; attaches to open business day automatically
  - DELETE /{id} — manager only; only allowed within the same open business day
  - GET /summary — totals by reason + by product for a period
- product_performance report: waste_qty + waste_cost added per product from the same period;
  products with only waste (no sales) also included
- Migration: CREATE TABLE IF NOT EXISTS waste_log

Frontend:
- WastePage (/waste): fast log form (product picker, quantity, reason chips, notes);
  today's entries list below (scoped to active business day); cost shown per entry;
  delete button for today's entries; history section behind toggle with date-range picker
- ProductPerformance report: Απόβλητα column showing waste_qty + cost in amber
- Sidebar: Trash2 icon for Αποβλήτα (after Καρτέλες)

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

local_backend/schemas/waste.py

@@ -0,0 +1,37 @@
+from pydantic import BaseModel
+from typing import Optional
+from datetime import datetime
+
+VALID_REASONS = {"kitchen_error", "dropped", "expired", "other"}
+
+REASON_LABELS = {
+    "kitchen_error": "Λάθος κουζίνας",
+    "dropped":       "Έπεσε",
+    "expired":       "Έληξε",
+    "other":         "Άλλο",
+}
+
+
+class WasteLogCreate(BaseModel):
+    product_id: int
+    quantity: float
+    reason: str
+    reason_notes: Optional[str] = None
+
+
+class WasteLogOut(BaseModel):
+    id: int
+    product_id: int
+    product_name: Optional[str] = None
+    quantity: float
+    unit_cost_snapshot: Optional[float] = None
+    total_cost: Optional[float] = None
+    reason: str
+    reason_label: Optional[str] = None
+    reason_notes: Optional[str] = None
+    logged_by_id: int
+    logged_by_name: Optional[str] = None
+    business_day_id: Optional[int] = None
+    logged_at: datetime
+
+    model_config = {"from_attributes": True}