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/schemas/schedule.py

@@ -0,0 +1,35 @@
+from pydantic import BaseModel
+from typing import Optional
+from datetime import date, datetime
+
+
+class ScheduledShiftCreate(BaseModel):
+    user_id: int
+    scheduled_date: date
+    start_time: str   # "HH:MM"
+    end_time: str     # "HH:MM"
+    notes: Optional[str] = None
+
+
+class ScheduledShiftUpdate(BaseModel):
+    start_time: Optional[str] = None
+    end_time: Optional[str] = None
+    notes: Optional[str] = None
+
+
+class ScheduledShiftOut(BaseModel):
+    id: int
+    user_id: int
+    user_name: Optional[str] = None
+    hourly_rate: Optional[float] = None
+    scheduled_date: date
+    start_time: str
+    end_time: str
+    notes: Optional[str] = None
+    created_by_id: int
+    created_at: datetime
+    # computed
+    duration_hours: Optional[float] = None
+    estimated_pay: Optional[float] = None
+
+    model_config = {"from_attributes": True}