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/migrations/sql/0008_savedata_hash.sql

@@ -0,0 +1,4 @@
+-- Add SHA-256 checksum column for savedata integrity verification.
+-- Stored as 32 raw bytes (not hex). NULL means no hash computed yet
+-- (backwards-compatible with existing data).
+ALTER TABLE characters ADD COLUMN IF NOT EXISTS savedata_hash BYTEA;