fix(schema): Database connection not cleanly closed

This commit introduces closing the Schema Manager's underlying driver
connection. We were only releasing the shared lock but not actually
cleaning up the db connection and relied on the fact that the process
would exit to do that for us.

internal/db/schema/internal/postgres/postgres.go

@@ -251,6 +251,11 @@ func (p *Postgres) Run(ctx context.Context, migration io.Reader, version int, ed
 	return nil
 }
 
+// Close closes the underlying Postgres database connection.
+func (p *Postgres) Close() error {
+	return p.conn.Close()
+}
+
 var errOldMigrationTable = stderrors.New("old schema migration table")
 
 func (p *Postgres) schemaInitialized(ctx context.Context) (bool, error) {