db: fix migration for existing schema_migrations without checksum column

- Add 0007 migration to ALTER TABLE adding checksum column
- Make migration runner INSERT conditional: if checksum col exists, use it; otherwise omit
- Handles existing databases (no checksum col) and fresh installs (has checksum col) correctly

internal/db/migrations/0007_schema_migrations_checksum.sql

@@ -0,0 +1,7 @@
+-- Migration 0007: Add checksum column to schema_migrations.
+-- This migration is idempotent and safe to re-run.
+-- For existing databases (pre-1.0.54): ALTER TABLE adds the checksum column.
+-- For fresh databases (1.0.54+): 0001_init.sql now creates the table with checksum column.
+-- This migration handles the upgrade path only.
+
+ALTER TABLE schema_migrations ADD COLUMN checksum TEXT DEFAULT '';