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/models/schedule.py

@@ -0,0 +1,24 @@
+from sqlalchemy import Column, Integer, String, Date, Time, Text, DateTime, ForeignKey
+from sqlalchemy.orm import relationship
+from datetime import datetime, timezone
+from database import Base
+
+
+def _utcnow():
+    return datetime.now(timezone.utc)
+
+
+class ScheduledShift(Base):
+    __tablename__ = "scheduled_shifts"
+
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    scheduled_date = Column(Date, nullable=False)
+    start_time = Column(String, nullable=False)  # "HH:MM" stored as string (SQLite has no Time type)
+    end_time = Column(String, nullable=False)
+    notes = Column(Text, nullable=True)
+    created_by_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    created_at = Column(DateTime(timezone=True), default=_utcnow)
+
+    user = relationship("User", foreign_keys=[user_id])
+    created_by = relationship("User", foreign_keys=[created_by_id])