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.

server/channelserver/handlers_character.go

@@ -55,7 +55,8 @@ func GetCharacterSaveData(s *Session, charID uint32) (*CharacterSaveData, error)
 	// Verify integrity checksum if one was stored with this save.
 	// A nil hash means the character was saved before checksums were introduced,
 	// so we skip verification (the next save will compute and store the hash).
-	if storedHash != nil {
+	// DisableSaveIntegrityCheck bypasses this entirely for cross-server save transfers.
+	if storedHash != nil && !s.server.erupeConfig.DisableSaveIntegrityCheck {
 		computedHash := sha256.Sum256(saveData.decompSave)
 		if !bytes.Equal(storedHash, computedHash[:]) {
 			s.logger.Error("Savedata integrity check failed: hash mismatch",
@@ -66,6 +67,10 @@ func GetCharacterSaveData(s *Session, charID uint32) (*CharacterSaveData, error)
 			// TODO: attempt recovery from savedata_backups here
 			return nil, errors.New("savedata integrity check failed")
 		}
+	} else if storedHash != nil && s.server.erupeConfig.DisableSaveIntegrityCheck {
+		s.logger.Warn("Savedata integrity check skipped (DisableSaveIntegrityCheck=true)",
+			zap.Uint32("charID", charID),
+		)
 	}
 
 	saveData.updateStructWithSaveData()

server/channelserver/handlers_character_test.go

@@ -747,3 +747,68 @@ func TestGetCharacterSaveData_ConfigMode(t *testing.T) {
 		})
 	}
 }
+
+// TestGetCharacterSaveData_IntegrityCheck verifies the SHA-256 hash guard and
+// that DisableSaveIntegrityCheck bypasses it without returning an error.
+func TestGetCharacterSaveData_IntegrityCheck(t *testing.T) {
+	// Build a minimal valid savedata blob and compress it.
+	rawSave := make([]byte, 150000)
+	copy(rawSave[88:], []byte("TestChar\x00"))
+	compressed, err := nullcomp.Compress(rawSave)
+	if err != nil {
+		t.Fatalf("compress: %v", err)
+	}
+
+	// A hash that deliberately does NOT match rawSave.
+	wrongHash := bytes.Repeat([]byte{0xDE}, 32)
+
+	tests := []struct {
+		name    string
+		disable bool
+		hash    []byte
+		wantErr bool
+	}{
+		{
+			name:    "nil hash skips check",
+			disable: false,
+			hash:    nil,
+			wantErr: false,
+		},
+		{
+			name:    "mismatched hash fails when check enabled",
+			disable: false,
+			hash:    wrongHash,
+			wantErr: true,
+		},
+		{
+			name:    "mismatched hash passes when check disabled",
+			disable: true,
+			hash:    wrongHash,
+			wantErr: false,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			mock := newMockCharacterRepo()
+			mock.loadSaveDataID = 1
+			mock.loadSaveDataData = compressed
+			mock.loadSaveDataName = "TestChar"
+			mock.loadSaveDataHash = tc.hash
+
+			server := createMockServer()
+			server.erupeConfig.RealClientMode = cfg.ZZ
+			server.erupeConfig.DisableSaveIntegrityCheck = tc.disable
+			server.charRepo = mock
+			session := createMockSession(1, server)
+
+			_, err := GetCharacterSaveData(session, 1)
+			if tc.wantErr && err == nil {
+				t.Error("expected error, got nil")
+			}
+			if !tc.wantErr && err != nil {
+				t.Errorf("unexpected error: %v", err)
+			}
+		})
+	}
+}

server/channelserver/repo_mocks_test.go

@@ -133,6 +133,7 @@ type mockCharacterRepo struct {
 	loadSaveDataData []byte
 	loadSaveDataNew  bool
 	loadSaveDataName string
+	loadSaveDataHash []byte
 	loadSaveDataErr  error
 
 	// ReadEtcPoints mock fields
@@ -245,7 +246,7 @@ func (m *mockCharacterRepo) SaveBackup(_ uint32, _ int, _ []byte) error       {
 func (m *mockCharacterRepo) GetLastBackupTime(_ uint32) (time.Time, error)    { return time.Time{}, nil }
 func (m *mockCharacterRepo) SaveCharacterDataAtomic(_ SaveAtomicParams) error { return nil }
 func (m *mockCharacterRepo) LoadSaveDataWithHash(_ uint32) (uint32, []byte, bool, string, []byte, error) {
-	return m.loadSaveDataID, m.loadSaveDataData, m.loadSaveDataNew, m.loadSaveDataName, nil, m.loadSaveDataErr
+	return m.loadSaveDataID, m.loadSaveDataData, m.loadSaveDataNew, m.loadSaveDataName, m.loadSaveDataHash, m.loadSaveDataErr
 }
 
 // --- mockGoocooRepo ---