feat: Phase 2J — staff shift scheduling

Backend:
- New model: ScheduledShift (scheduled_shifts table)
  Planning data separate from waiter_shifts (actual data) — linked conceptually by user+date
  start_time/end_time stored as "HH:MM" strings (SQLite has no native Time type)
- New router /api/schedule/:
  - GET / — list by date range + optional user_id filter
  - POST / — create scheduled shift (validates user exists)
  - PUT /{id} — update times/notes
  - DELETE /{id} — remove
  - GET /week?week_start= — full week view: scheduled shifts with actual WaiterShift
    comparison for the same user+date, estimated weekly labor cost, waiters list with rates
- Each ScheduledShiftOut includes duration_hours (handles overnight) + estimated_pay
  (duration × hourly_rate, null if rate not set)
- Migration: CREATE TABLE IF NOT EXISTS scheduled_shifts

Frontend:
- SchedulePage (/schedule): weekly calendar grid — rows = staff, columns = Mon–Sun
  - Week navigation (← Προηγ. / Αυτή η εβδομάδα / Επόμ. →)
  - Estimated weekly labor cost in header when shifts have rates
  - ShiftSlot cells: blue = scheduled only, green = scheduled + actual shift happened
    Shows scheduled times + actual start/end from real WaiterShift data
  - Click empty cell → AddShiftModal (pre-selects that row's waiter, any date in week)
  - "+" row at bottom for waiters not yet scheduled this week
  - Delete button (✕) per slot with confirm
  - Legend explaining the color scheme
- Sidebar: CalendarDays icon for Πρόγραμμα (after KDS)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/main.py

@@ -25,6 +25,7 @@ import models.expenses       # noqa: F401 — registers Contact, Expense, Expens
 import models.customers      # noqa: F401 — registers Customer
 import models.tabs           # noqa: F401 — registers Tab, TabEntry, TabPayment
 import models.waste          # noqa: F401 — registers WasteLog
+import models.schedule       # noqa: F401 — registers ScheduledShift
 
 from routers import auth, tables, products, orders, waiters, reports, system, setup as setup_router
 from routers import business_day as business_day_router
@@ -42,6 +43,7 @@ from routers import customers as customers_router
 from routers import tabs as tabs_router
 from routers import waste as waste_router
 from routers import kds as kds_router
+from routers import schedule as schedule_router
 
 
 def _run_migrations():
@@ -280,6 +282,17 @@ def _run_migrations():
         # Phase 2B — staff payroll
         "ALTER TABLE users ADD COLUMN hourly_rate REAL",
         "ALTER TABLE waiter_shifts ADD COLUMN hourly_rate_snapshot REAL",
+        # Phase 2J — staff shift scheduling
+        """CREATE TABLE IF NOT EXISTS scheduled_shifts (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            user_id INTEGER NOT NULL REFERENCES users(id),
+            scheduled_date DATE NOT NULL,
+            start_time VARCHAR NOT NULL,
+            end_time VARCHAR NOT NULL,
+            notes TEXT,
+            created_by_id INTEGER NOT NULL REFERENCES users(id),
+            created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+        )""",
         # Phase 2H — void/waste log
         """CREATE TABLE IF NOT EXISTS waste_log (
             id INTEGER PRIMARY KEY AUTOINCREMENT,
@@ -462,3 +475,4 @@ app.include_router(customers_router.router,      prefix="/api/customers",    tag
 app.include_router(tabs_router.router,           prefix="/api/tabs",         tags=["tabs"])
 app.include_router(waste_router.router,          prefix="/api/waste",        tags=["waste"])
 app.include_router(kds_router.router,            prefix="/api/kds",          tags=["kds"])
+app.include_router(schedule_router.router,       prefix="/api/schedule",     tags=["schedule"])