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/shifts.py

@@ -35,10 +35,46 @@ def compute_shift_total(shift_id: int, db: Session) -> float:
     return round(sum(i.unit_price * i.quantity for i in items), 2)
 
 
+def compute_shift_pay(shift: WaiterShift) -> dict:
+    """Return duration_hours and shift_pay. shift_pay is None if no rate snapshot."""
+    now = datetime.now(timezone.utc)
+    start = shift.started_at
+    if start and start.tzinfo is None:
+        start = start.replace(tzinfo=timezone.utc)
+    end = shift.ended_at
+    if end and end.tzinfo is None:
+        end = end.replace(tzinfo=timezone.utc)
+    reference = end or now
+
+    total_seconds = (reference - start).total_seconds() if start else 0
+
+    # Subtract completed breaks
+    break_seconds = 0
+    for b in shift.breaks:
+        bs = b.started_at
+        be = b.ended_at
+        if bs and be:
+            if bs.tzinfo is None:
+                bs = bs.replace(tzinfo=timezone.utc)
+            if be.tzinfo is None:
+                be = be.replace(tzinfo=timezone.utc)
+            break_seconds += (be - bs).total_seconds()
+
+    worked_seconds = max(0, total_seconds - break_seconds)
+    duration_hours = round(worked_seconds / 3600, 4)
+
+    shift_pay = None
+    if shift.hourly_rate_snapshot is not None:
+        shift_pay = round(duration_hours * shift.hourly_rate_snapshot, 2)
+
+    return {"duration_hours": round(duration_hours, 2), "shift_pay": shift_pay}
+
+
 def _enrich_shift(shift: WaiterShift, db: Session) -> dict:
     w = shift.waiter
     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)
+    pay_data = compute_shift_pay(shift)
     return {
         "id": shift.id,
         "waiter_id": shift.waiter_id,
@@ -51,6 +87,9 @@ def _enrich_shift(shift: WaiterShift, db: Session) -> dict:
         "net_to_deliver": round(total + (shift.starting_cash or 0.0), 2),
         "is_active": shift.ended_at is None,
         "notes": shift.notes,
+        "hourly_rate_snapshot": shift.hourly_rate_snapshot,
+        "duration_hours": pay_data["duration_hours"],
+        "shift_pay": pay_data["shift_pay"],
         "breaks": [
             {"id": b.id, "shift_id": b.shift_id, "started_at": _dt(b.started_at), "ended_at": _dt(b.ended_at)}
             for b in shift.breaks
@@ -97,10 +136,12 @@ def start_shift(
     if existing:
         raise HTTPException(status_code=400, detail="Waiter already has an active shift")
 
+    target_user = db.query(User).filter(User.id == target_id).first()
     shift = WaiterShift(
         waiter_id=target_id,
         business_day_id=active_day.id,
         starting_cash=body.starting_cash,
+        hourly_rate_snapshot=target_user.hourly_rate if target_user else None,
     )
     db.add(shift)
     db.commit()
@@ -169,6 +210,7 @@ def manager_start_shift(
         waiter_id=body.waiter_id,
         business_day_id=active_day.id,
         starting_cash=body.starting_cash,
+        hourly_rate_snapshot=target.hourly_rate,
     )
     db.add(shift)
     db.commit()