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.

internal/database/schema/mysql/migrations/1_bug_reports.sql

@@ -0,0 +1,4 @@
+ALTER TABLE bug_reports ADD COLUMN hostname VARCHAR(255) NOT NULL DEFAULT '' AFTER hwid;
+ALTER TABLE bug_reports ADD COLUMN os_user VARCHAR(255) NOT NULL DEFAULT '' AFTER hostname;
+ALTER TABLE bug_reports ADD INDEX idx_hostname (hostname);
+ALTER TABLE bug_reports ADD INDEX idx_os_user (os_user);