db: make migrations a struct instead of a plain string

pkg/db/migration.go

@@ -0,0 +1,12 @@
+package db
+
+type Migration struct {
+	// ID is the version number that the schema will be at, after applying the migration.
+	ID int
+
+	// Title is an optional name for the migration.
+	Title string
+
+	// SQL is the body of the migration that will be executed.
+	SQL string
+}