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/Core/Services/StadiumProximityMatcher.swift

@@ -60,7 +60,7 @@ extension MatchConfidence: Comparable {
 // MARK: - Stadium Match
 
 struct StadiumMatch: Identifiable, Sendable {
-    let id: UUID
+    let id: String
     let stadium: Stadium
     let distance: CLLocationDistance
     let confidence: MatchConfidence