feat: Phase 2B — staff payroll / hourly rate

- Users: add hourly_rate (Float, nullable) — manager-only, not shown to waiter
- WaiterShift: add hourly_rate_snapshot — copied from user.hourly_rate at shift open, never updated
- Shift start (both /start and /manager/start): snapshot rate at shift creation
- compute_shift_pay(): calculates worked hours (minus breaks) × rate_snapshot; returns None if unset
- _enrich_shift(): now includes hourly_rate_snapshot, duration_hours, shift_pay
- shifts_report in reports.py: delegates to _enrich_shift so all shift endpoints are consistent
- Shifts CSV export: adds duration_hours, hourly_rate, shift_pay columns
- StaffTab: hourly_rate field in waiter edit modal (Μισθοδοσία section, col 3)
- ShiftsOverview report: Αμοιβή/ώρα + Αποδοχές columns; untracked shows 'Δεν παρακολουθείται'

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

local_backend/routers/reports.py

@@ -872,7 +872,7 @@ def shifts_report(
     db: Session = Depends(get_db),
     user: User = Depends(require_manager),
 ):
-    from routers.shifts import compute_shift_total
+    from routers.shifts import _enrich_shift
 
     q = db.query(WaiterShift)
     if waiter_id:
@@ -887,28 +887,7 @@ def shifts_report(
         q = q.filter(WaiterShift.ended_at == None)
 
     shifts = q.order_by(WaiterShift.started_at.desc()).all()
-    waiters_db = {u.id: u for u in db.query(User).all()}
-
-    result = []
-    for shift in shifts:
-        w = waiters_db.get(shift.waiter_id)
-        wname = (w.full_name or w.username) if w else f"#{shift.waiter_id}"
-        total = compute_shift_total(shift.id, db) if shift.ended_at is None else (shift.total_collected or 0.0)
-        result.append({
-            "id": shift.id,
-            "waiter_id": shift.waiter_id,
-            "waiter_name": wname,
-            "business_day_id": shift.business_day_id,
-            "started_at": _dt(shift.started_at),
-            "ended_at": _dt(shift.ended_at),
-            "starting_cash": shift.starting_cash,
-            "total_collected": total,
-            "net_to_deliver": round(total + (shift.starting_cash or 0.0), 2),
-            "is_active": shift.ended_at is None,
-            "notes": shift.notes,
-        })
-
-    return {"shifts": result}
+    return {"shifts": [_enrich_shift(s, db) for s in shifts]}
 
 
 # ---------------------------------------------------------------------------
@@ -1513,6 +1492,9 @@ def shifts_export(
             "waiter": s["waiter_name"],
             "started_at": s["started_at"],
             "ended_at": s["ended_at"] or "",
+            "duration_hours": s.get("duration_hours", ""),
+            "hourly_rate": s.get("hourly_rate_snapshot", ""),
+            "shift_pay": s.get("shift_pay", ""),
             "starting_cash": s["starting_cash"],
             "total_collected": s["total_collected"],
             "net_to_deliver": s["net_to_deliver"],