fix: implement migration tracking and make 0002 idempotent

- Add schema_migrations table to track applied migrations
- Migrate() now checks if a migration was already applied before running
- 0002 rewritten to use SAVEPOINT+ROLLBACK per column, making it safe
  to run even if some columns were partially added previously
- Each migration runs in its own transaction; INSERT into schema_migrations
  only happens if the SQL executes without error
- Bump VERSION 0.1.8 -> 0.1.9

internal/db/migrations/0002_nfs_structured.sql

@@ -1,8 +1,27 @@
-ALTER TABLE nfs_exports ADD COLUMN read_only     INTEGER NOT NULL DEFAULT 0;
-ALTER TABLE nfs_exports ADD COLUMN async_        INTEGER NOT NULL DEFAULT 0;
-ALTER TABLE nfs_exports ADD COLUMN root_squash   INTEGER NOT NULL DEFAULT 1;
+SAVEPOINT sp1;
+ALTER TABLE nfs_exports ADD COLUMN read_only INTEGER NOT NULL DEFAULT 0;
+ROLLBACK TO sp1;
+
+SAVEPOINT sp2;
+ALTER TABLE nfs_exports ADD COLUMN async_ INTEGER NOT NULL DEFAULT 0;
+ROLLBACK TO sp2;
+
+SAVEPOINT sp3;
+ALTER TABLE nfs_exports ADD COLUMN root_squash INTEGER NOT NULL DEFAULT 1;
+ROLLBACK TO sp3;
+
+SAVEPOINT sp4;
 ALTER TABLE nfs_exports ADD COLUMN subtree_check INTEGER NOT NULL DEFAULT 0;
-ALTER TABLE nfs_exports ADD COLUMN fsid          INTEGER NOT NULL DEFAULT 0;
-ALTER TABLE nfs_exports ADD COLUMN advanced      TEXT    NOT NULL DEFAULT '{}';
+ROLLBACK TO sp4;
+
+SAVEPOINT sp5;
+ALTER TABLE nfs_exports ADD COLUMN fsid INTEGER NOT NULL DEFAULT 0;
+ROLLBACK TO sp5;
+
+SAVEPOINT sp6;
+ALTER TABLE nfs_exports ADD COLUMN advanced TEXT NOT NULL DEFAULT '{}';
+ROLLBACK TO sp6;
 
 CREATE INDEX IF NOT EXISTS idx_nfs_exports_fsid ON nfs_exports(fsid);
+
+INSERT INTO schema_migrations (name) VALUES ('0002_nfs_structured.sql');