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>

SportsTimeTests/Mocks/MockCloudKitService.swift

@@ -155,7 +155,7 @@ actor MockCloudKitService {
         }.sorted { $0.dateTime < $1.dateTime }
     }
 
-    func fetchGame(by id: UUID) async throws -> Game? {
+    func fetchGame(by id: String) async throws -> Game? {
         try await simulateNetwork()
         return games.first { $0.id == id }
     }
@@ -167,7 +167,7 @@ actor MockCloudKitService {
         return stadiums.map { stadium in
             CloudKitService.SyncStadium(
                 stadium: stadium,
-                canonicalId: stadium.id.uuidString
+                canonicalId: stadium.id
             )
         }
     }
@@ -177,8 +177,8 @@ actor MockCloudKitService {
         return teams.filter { $0.sport == sport }.map { team in
             CloudKitService.SyncTeam(
                 team: team,
-                canonicalId: team.id.uuidString,
-                stadiumCanonicalId: team.stadiumId.uuidString
+                canonicalId: team.id,
+                stadiumCanonicalId: team.stadiumId
             )
         }
     }
@@ -197,10 +197,10 @@ actor MockCloudKitService {
         }.map { game in
             CloudKitService.SyncGame(
                 game: game,
-                canonicalId: game.id.uuidString,
-                homeTeamCanonicalId: game.homeTeamId.uuidString,
-                awayTeamCanonicalId: game.awayTeamId.uuidString,
-                stadiumCanonicalId: game.stadiumId.uuidString
+                canonicalId: game.id,
+                homeTeamCanonicalId: game.homeTeamId,
+                awayTeamCanonicalId: game.awayTeamId,
+                stadiumCanonicalId: game.stadiumId
             )
         }
     }