feat: staff work schedule — weekly grid for all roles, shift assignment, day off marking

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

backend/migrations/037_staff_schedule.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS staff_schedules (
+  id          UUID        PRIMARY KEY DEFAULT gen_random_uuid(),
+  hotel_id    UUID        NOT NULL REFERENCES hotels(id) ON DELETE CASCADE,
+  user_id     UUID        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  date        DATE        NOT NULL,
+  shift_start TIME,
+  shift_end   TIME,
+  is_day_off  BOOLEAN     NOT NULL DEFAULT false,
+  notes       TEXT,
+  created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  UNIQUE(hotel_id, user_id, date)
+);