add support for SQLite as an alternative database backend

Implement SQLite support using the pure Go `modernc.org/sqlite` driver and update the migration system to handle driver-specific schemas. Users can now choose between MySQL and SQLite by setting the `DB_DRIVER` environment variable.

go.mod

@@ -10,7 +10,18 @@ require (
 	golang.org/x/crypto v0.49.0
 )
 
-require golang.org/x/sys v0.42.0 // indirect
+require (
+	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/ncruces/go-strftime v1.0.0 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	golang.org/x/sys v0.42.0 // indirect
+	modernc.org/libc v1.70.0 // indirect
+	modernc.org/mathutil v1.7.1 // indirect
+	modernc.org/memory v1.11.0 // indirect
+	modernc.org/sqlite v1.47.0 // indirect
+)
 
 require (
 	filippo.io/edwards25519 v1.1.1 // indirect