docs: log savedata corruption defense in CHANGELOG

Merge migrations 0007 and 0008 into a single 0007_savedata_integrity
migration since neither has been released yet.

server/migrations/sql/0007_savedata_integrity.sql

@@ -0,0 +1,16 @@
+-- Savedata integrity protections: rotating backups + checksum verification.
+
+-- Rotating backup table (3 slots per character, time-gated).
+-- Prevents permanent data loss from save corruption by keeping recent snapshots.
+CREATE TABLE IF NOT EXISTS savedata_backups (
+    char_id     INTEGER NOT NULL REFERENCES characters(id) ON DELETE CASCADE,
+    slot        SMALLINT NOT NULL CHECK (slot BETWEEN 0 AND 2),
+    savedata    BYTEA NOT NULL,
+    saved_at    TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (char_id, slot)
+);
+
+-- 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;