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/config_test.go

@@ -157,7 +157,8 @@ func TestConfigStructTypes(t *testing.T) {
 		LoginNotices:           []string{"Notice"},
 		PatchServerManifest:    "http://patch.example.com",
 		PatchServerFile:        "http://files.example.com",
-		DeleteOnSaveCorruption: false,
+		DeleteOnSaveCorruption:    false,
+		DisableSaveIntegrityCheck: false,
 		ClientMode:             "ZZ",
 		RealClientMode:         ZZ,
 		QuestCacheExpiry:       3600,