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/schemas/user.py

@@ -14,6 +14,8 @@ class UserBase(BaseModel):
     note: Optional[str] = None
     avatar_url: Optional[str] = None
     email: Optional[str] = None
+    # Phase 2B — payroll
+    hourly_rate: Optional[float] = None
 
 
 class UserCreate(UserBase):
@@ -29,6 +31,8 @@ class UserUpdate(BaseModel):
     mobile_phone: Optional[str] = None
     email: Optional[str] = None
     note: Optional[str] = None
+    # Phase 2B — payroll
+    hourly_rate: Optional[float] = None
 
 
 class WaiterZoneOut(BaseModel):