feat(02-01): add migration infrastructure with golang-migrate and embedded SQL

- RunMigrations applies versioned SQL files via golang-migrate + embed.FS (iofs)
- ErrNoChange handled correctly - not treated as failure
- Migration 0001 creates full current schema with CREATE TABLE IF NOT EXISTS
- All three tables (updates, tags, tag_assignments) with acknowledged_at and ON DELETE CASCADE
- Uses database/sqlite sub-package (modernc.org/sqlite, no CGO)
- go mod tidy applied after adding dependencies

pkg/diunwebhook/migrations/sqlite/0001_initial_schema.down.sql

@@ -0,0 +1,3 @@
+DROP TABLE IF EXISTS tag_assignments;
+DROP TABLE IF EXISTS tags;
+DROP TABLE IF EXISTS updates;