feat(itinerary): add draggable travel day positioning with CloudKit persistence

- Add TravelDayOverride model for storing user-customized travel day positions
- Add TravelOverrideService for CloudKit CRUD operations on travel overrides
- Add CKTravelDayOverride CloudKit model wrapper
- Refactor itinerarySections to validate travel day bounds (must be after last game in departure city)
- Travel segments can now be dragged to different days within valid range
- Persist travel day overrides to CloudKit for cross-device sync

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Core/Models/Domain/TravelDayOverride.swift

@@ -0,0 +1,38 @@
+//
+//  TravelDayOverride.swift
+//  SportsTime
+//
+//  Stores user-customized travel day positions
+//
+
+import Foundation
+
+/// Represents a user override of the default travel day placement.
+/// Travel segments normally appear on the day after the last game in the departure city.
+/// This model allows users to drag travel to a different day within valid bounds.
+struct TravelDayOverride: Identifiable, Codable, Hashable {
+    let id: UUID
+    let tripId: UUID
+    /// Stable identifier for the travel segment (e.g., "travel:houston->arlington")
+    let travelAnchorId: String
+    /// The day number (1-indexed) where the travel should display
+    var displayDay: Int
+    let createdAt: Date
+    var modifiedAt: Date
+
+    init(
+        id: UUID = UUID(),
+        tripId: UUID,
+        travelAnchorId: String,
+        displayDay: Int,
+        createdAt: Date = Date(),
+        modifiedAt: Date = Date()
+    ) {
+        self.id = id
+        self.tripId = tripId
+        self.travelAnchorId = travelAnchorId
+        self.displayDay = displayDay
+        self.createdAt = createdAt
+        self.modifiedAt = modifiedAt
+    }
+}