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/user.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Column, Integer, String, Boolean, DateTime, ForeignKey
+from sqlalchemy import Column, Integer, String, Boolean, DateTime, Float, ForeignKey
 from sqlalchemy.orm import relationship
 from datetime import datetime, timezone
 from database import Base
@@ -24,6 +24,8 @@ class User(Base):
     note = Column(String, nullable=True)
     avatar_url = Column(String, nullable=True)
     created_at = Column(DateTime(timezone=True), default=_utcnow)
+    # Phase 2B — payroll
+    hourly_rate = Column(Float, nullable=True)
 
     orders_opened = relationship("Order", foreign_keys="Order.opened_by", back_populates="opener")
     orders_closed = relationship("Order", foreign_keys="Order.closed_by", back_populates="closer")