feat: implement SQLite backup & data integrity plan (Phases 2-4)

app/migrations/applied/026_audit_log.sql

@@ -0,0 +1,16 @@
+-- Migration 026: create audit_log table for data-level audit trail
+-- Records before/after snapshots of every row mutation on core tables.
+-- Admin actions are already logged separately via admin_audit_log.
+CREATE TABLE IF NOT EXISTS audit_log (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    timestamp   TEXT    NOT NULL DEFAULT (datetime('now')),
+    actor       TEXT    NOT NULL,
+    action      TEXT    NOT NULL CHECK(action IN ('INSERT','UPDATE','DELETE')),
+    table_name  TEXT    NOT NULL,
+    record_id   INTEGER,
+    old_data    TEXT,
+    new_data    TEXT
+);
+
+CREATE INDEX IF NOT EXISTS idx_audit_log_table_record ON audit_log(table_name, record_id);
+CREATE INDEX IF NOT EXISTS idx_audit_log_timestamp      ON audit_log(timestamp);