refactor: change domain model IDs from UUID to String canonical IDs

This refactor fixes the achievement system by using stable canonical string
IDs (e.g., "stadium_mlb_fenway_park") instead of random UUIDs. This ensures
stadium mappings for achievements are consistent across app launches and
CloudKit sync operations.

Changes:
- Stadium, Team, Game: id property changed from UUID to String
- Trip, TripStop, TripPreferences: updated to use String IDs for games/stadiums
- CKModels: removed UUID parsing, use canonical IDs directly
- AchievementEngine: now matches against canonical stadium IDs
- All test files updated to use String IDs instead of UUID()

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

SportsTime/Features/Trip/ViewModels/TripCreationViewModel.swift

@@ -108,7 +108,7 @@ final class TripCreationViewModel {
     }
 
     // Games
-    var mustSeeGameIds: Set<UUID> = []
+    var mustSeeGameIds: Set<String> = []
     var availableGames: [RichGame] = []
     var isLoadingGames: Bool = false
 
@@ -134,7 +134,7 @@ final class TripCreationViewModel {
     var selectedRegions: Set<Region> = [.east, .central, .west]
 
     // Follow Team Mode
-    var followTeamId: UUID?
+    var followTeamId: String?
     var useHomeLocation: Bool = true
 
     // Game First Mode - Trip duration for sliding windows
@@ -148,8 +148,8 @@ final class TripCreationViewModel {
 
     // MARK: - Cached Data
 
-    private var teams: [UUID: Team] = [:]
-    private var stadiums: [UUID: Stadium] = [:]
+    private var teams: [String: Team] = [:]
+    private var stadiums: [String: Stadium] = [:]
     private var games: [Game] = []
     private(set) var currentPreferences: TripPreferences?
 
@@ -454,7 +454,7 @@ final class TripCreationViewModel {
         }
     }
 
-    func toggleMustSeeGame(_ gameId: UUID) {
+    func toggleMustSeeGame(_ gameId: String) {
         if mustSeeGameIds.contains(gameId) {
             mustSeeGameIds.remove(gameId)
         } else {