feat(savedata): add tier 2 integrity protections

Strengthen savedata persistence against corruption and race conditions:

- SHA-256 checksum: hash the decompressed blob on every save, store in
  new savedata_hash column, verify on load to detect silent corruption.
  Pre-existing characters with no hash are silently upgraded on next save.
- Atomic transactions: wrap character data + house data + hash + backup
  into a single DB transaction via SaveCharacterDataAtomic, so a crash
  mid-save never leaves partial state.
- Per-character save mutex: CharacterLocks (sync.Map of charID → Mutex)
  serializes concurrent saves for the same character, preventing races
  that could defeat corruption detection. Different characters remain
  fully independent.

Migration 0008 adds the savedata_hash column to the characters table.

server/channelserver/handlers_data.go

@@ -28,6 +28,11 @@ const (
 func handleMsgMhfSavedata(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgMhfSavedata)
 
+	// Serialize saves for the same character to prevent concurrent operations
+	// from racing and defeating corruption detection.
+	unlock := s.server.charSaveLocks.Lock(s.charID)
+	defer unlock()
+
 	if len(pkt.RawDataPayload) > saveDataMaxCompressedPayload {
 		s.logger.Warn("Savedata payload exceeds size limit",
 			zap.Int("len", len(pkt.RawDataPayload)),