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/main.py

@@ -266,6 +266,9 @@ def _run_migrations():
         "ALTER TABLE products ADD COLUMN cost_simple REAL",
         "ALTER TABLE products ADD COLUMN cost_breakdown TEXT",
         "ALTER TABLE order_items ADD COLUMN unit_cost REAL",
+        # Phase 2B — staff payroll
+        "ALTER TABLE users ADD COLUMN hourly_rate REAL",
+        "ALTER TABLE waiter_shifts ADD COLUMN hourly_rate_snapshot REAL",
     ]
     for sql in migrations:
         try: