fix: prevent idle transaction timeout in postgresql during tunnel update (#404)

## Summary
- Moved `tx := h.repo.BeginTx()` below `applyFederationRuntime` in
`tunnelUpdate`
- This prevents the database transaction from remaining idle for an
extended period of time while making HTTP network requests to federation
nodes, which can trigger the `idle_in_transaction_session_timeout` or
cause connection pool exhaustion in PostgreSQL.

## Test Plan
- [x] Backend tests passed (`go test ./...`)
- [ ] Verify tunnel updates no longer timeout with PostgreSQL

go-backend/internal/http/handler/mutations.go

@@ -778,14 +778,7 @@ func (h *Handler) tunnelUpdate(w http.ResponseWriter, r *http.Request) {
 	ipPreference := asString(req["ipPreference"])
 	localDomain := h.federationLocalDomain()
 
-	tx := h.repo.BeginTx()
+	runtimeState, err := h.prepareTunnelCreateState(h.repo.DB(), req, typeVal, id)
-	if tx.Error != nil {
-		response.WriteJSON(w, response.Err(-2, tx.Error.Error()))
-		return
-	}
-	defer func() { tx.Rollback() }()
-
-	runtimeState, err := h.prepareTunnelCreateState(tx, req, typeVal, id)
 	if err != nil {
 		response.WriteJSON(w, response.ErrDefault(err.Error()))
 		return
@@ -804,6 +797,14 @@ func (h *Handler) tunnelUpdate(w http.ResponseWriter, r *http.Request) {
 	}
 	applyTunnelPortsToRequest(req, runtimeState)
 
+	tx := h.repo.BeginTx()
+	if tx.Error != nil {
+		h.releaseFederationRuntimeRefs(federationReleaseRefs)
+		response.WriteJSON(w, response.Err(-2, tx.Error.Error()))
+		return
+	}
+	defer func() { tx.Rollback() }()
+
 	if err := h.repo.UpdateTunnelTx(
 		tx,
 		id,