feat: Phase 2E — cash drawer reconciliation

Backend:
- WaiterShift: add counted_cash_end (physical cash handed over) + cash_discrepancy (computed)
- BusinessDay: add store_opening_cash, store_closing_cash, store_cash_discrepancy
- EndShiftRequest: accept counted_cash_end (optional)
- CloseBusinessDayRequest: accept store_closing_cash (optional)
- Shift end (both /end and /manager/end/{id}): compute cash_discrepancy =
  counted_cash_end - (starting_cash + total_collected); negative = short, positive = over
- Business day close: compute store_cash_discrepancy = store_closing_cash - expected;
  expected = store_opening_cash + sum(shift.total_collected for day)
- _enrich_shift: expose counted_cash_end + cash_discrepancy in all shift responses
- Business day summary: expose store cash fields + store_expected_closing + total_waiter_collected
- BusinessDayOut schema: expose new store cash fields
- Shifts CSV export: add counted_cash_end + cash_discrepancy columns

Frontend:
- EndShiftConfirmModal: 2-step flow — step 1 notes, step 2 cash handover with live discrepancy
  (shows ✓ clean / ⚠ short / ⚠ over in real time before confirming)
- WorkdaySummaryModal close flow: new Ταμείο tab with store cash count + live discrepancy;
  footer navigates overview → Ταμείο → Έλεγχοι; store_closing_cash included in close payload
- ShiftsOverview report: Ταμείο column showing counted_cash_end + discrepancy badge
- ShiftDetailModal row 2: replaces Εξ.Τραπεζιών with Παραδόθηκαν + Ταμείο discrepancy

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

local_backend/schemas/business_day.py

@@ -11,6 +11,10 @@ class BusinessDayOut(BaseModel):
     closed_at: Optional[UTCDatetime] = None
     closed_by_id: Optional[int] = None
     notes: Optional[str] = None
+    # Phase 2E
+    store_opening_cash: Optional[float] = None
+    store_closing_cash: Optional[float] = None
+    store_cash_discrepancy: Optional[float] = None
 
     model_config = {"from_attributes": True}
 
@@ -22,3 +26,4 @@ class OpenBusinessDayRequest(BaseModel):
 class CloseBusinessDayRequest(BaseModel):
     force: bool = False
     notes: Optional[str] = None
+    store_closing_cash: Optional[float] = None  # Phase 2E: physical cash counted at close