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/sys_channel_server.go

@@ -96,6 +96,10 @@ type Server struct {
 	userBinary *UserBinaryStore
 	minidata   *MinidataStore
 
+	// Per-character save locks prevent concurrent save operations for the
+	// same character from racing and defeating corruption detection.
+	charSaveLocks CharacterLocks
+
 	// Semaphore
 	semaphoreLock  sync.RWMutex
 	semaphore      map[string]*Semaphore