Stage 5: on-call schedule

- Migration 005: schedule_entries table (date TEXT UNIQUE, one person per day)
- POST /api/schedule — assign user to one or more dates in a single
  transaction; any date conflict rejects the whole request (409)
- GET /api/schedule — list all entries ordered by date, optional ?from/?to
- GET /api/schedule/current — today's on-call user (UTC date), 404 if none
- DELETE /api/schedule/{id} — remove an entry (204)

internal/models/schedule.go

@@ -0,0 +1,11 @@
+package models
+
+import "time"
+
+type ScheduleEntry struct {
+	ID        int64     `json:"id"`
+	UserID    int64     `json:"user_id"`
+	Username  string    `json:"username"`
+	Date      string    `json:"date"` // YYYY-MM-DD
+	CreatedAt time.Time `json:"created_at"`
+}