Replace scs postgresstore with a sqlc-backed SessionStore.

Keep scs for cookies and session API while sessions DDL and queries live in the same sqlc stack as the rest of Postgres.

internal/store/postgres.go

@@ -67,10 +67,6 @@ func OpenPostgres(databaseURL, schema string) (*sql.DB, *SessionStore, error) {
 		_ = db.Close()
 		return nil, nil, fmt.Errorf("apply schema: %w", err)
 	}
-	if err := applySessionsSchema(db); err != nil {
-		_ = db.Close()
-		return nil, nil, fmt.Errorf("apply sessions schema: %w", err)
-	}
 	if err := migrateUserProfileColumns(db); err != nil {
 		_ = db.Close()
 		return nil, nil, fmt.Errorf("migrate profile columns: %w", err)