Phase 6 (Stufe 5): Default-Provider auf MySQL + Readback-Verifikation

- appsettings.json: Database:Provider = "MySql" (voller Umstieg).
- ConfigMigrator.VerifyMySql + CLI --verify-mysql: liest Accounts,
  Settings, Trader, Markets aus MySQL zurueck.
- Verifiziert: 3 Accounts, 3 Settings, 32 Trader (23 aktiv),
  2344 Markets; Werte/AssignedAccountIds stimmen mit Exporten ueberein.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

appsettings.json

@@ -1,6 +1,6 @@
 {
   "Database": {
-    "Provider": "Mongo",
+    "Provider": "MySql",
     "ConnectionString": "mongodb://localhost:27017",
     "DatabaseName": "PolyTraderDB"
   }