fix(store): pin connection pool so foreign_keys pragma sticks

SQLite PRAGMAs are connection-scoped, but database/sql uses a connection
pool. Without pinning to one connection, new pooled connections won't have
foreign_keys enabled, breaking ON DELETE CASCADE enforcement.

Also mark modernc.org/sqlite as a direct dependency in go.mod.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/store/store.go

@@ -27,6 +27,10 @@ func Open(path string, key []byte) (*Store, error) {
 	if err != nil {
 		return nil, err
 	}
+	// Pin pool to a single connection so PRAGMA foreign_keys = ON sticks.
+	// SQLite PRAGMAs are connection-scoped; pool would otherwise create
+	// new connections without the pragma set.
+	db.SetMaxOpenConns(1)
 	if _, err := db.Exec("PRAGMA foreign_keys = ON;"); err != nil {
 		db.Close()
 		return nil, err