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.
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