feat(config): add DisableSaveIntegrityCheck flag for save transfers

The SHA-256 integrity check introduced in migration 0007 blocks saves
when a character's savedata blob is imported from another server instance,
because the stored hash in the target DB no longer matches the new blob.

Adding DisableSaveIntegrityCheck (default: false) lets server operators
bypass the check to unblock cross-server save transfers. A warning is
logged each time the check is skipped so the flag's use is auditable.

Documents the per-character SQL alternative in CHANGELOG:
  UPDATE characters SET savedata_hash = NULL WHERE id = <id>

Closes #183.

config.reference.json

@@ -17,6 +17,7 @@
     "UploadQuality":100
   },
   "DeleteOnSaveCorruption": false,
+  "DisableSaveIntegrityCheck": false,
   "ClientMode": "ZZ",
   "QuestCacheExpiry": 300,
   "CommandPrefix": "!",