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/models/shift.py

@@ -19,6 +19,8 @@ class WaiterShift(Base):
     starting_cash = Column(Float, nullable=True)
     total_collected = Column(Float, nullable=True)  # snapshot written at shift end
     notes = Column(Text, nullable=True)
+    # Phase 2B — payroll snapshot (copied from user.hourly_rate at shift start, never updated)
+    hourly_rate_snapshot = Column(Float, nullable=True)
 
     waiter = relationship("User", foreign_keys=[waiter_id])
     business_day = relationship("BusinessDay", back_populates="shifts")