fix: add missing freeCharLimit and premiumCharLimit to TTSQuotaService

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: psypeal

App/Services/TTSQuotaService.swift

@@ -4,6 +4,11 @@ import SwiftData
 /// Tracks TTS generation statistics (count and characters used).
 @MainActor
 final class TTSQuotaService {
+    /// Daily character limit for free users.
+    static let freeCharLimit = 500
+    /// Daily character limit for premium users.
+    static let premiumCharLimit = 50_000
+
     /// Get today's character usage.
     func getUsageToday(modelContext: ModelContext) -> Int {
         let todayId = TTSUsageRecord.todayId()