Expose gateway billing flows from the synced server baseline

The preserved local work adds PopiNewAPI billing checkout, gateway account binding, and matching web console/pricing surfaces on top of the current test-server baseline. During the merge, the start-end video helpers from the deployed baseline were kept alongside the actionGenerate prompt handling from the WIP.

Constraint: Current usable baseline is 7054436, already deployed on the test server.

Rejected: Commit the WIP before syncing the baseline | would have hidden conflicts with the deployed start-end-frame changes.

Confidence: medium

Scope-risk: broad

Directive: Do not deploy this commit to the test server without rechecking gateway credentials and billing checkout behavior in that environment.

Tested: go test ./...

Tested: make build

Tested: cd web && npm run build

internal/server/store.go

@@ -152,6 +152,10 @@ func (s *store) rotateSession(oldToken string) (string, user, bool, error) {
 	return current.Token, current.User, true, nil
 }
 
+func (s *store) bindGatewaySession(token string, gatewayUserID int, gatewayAccessToken string) error {
+	return s.sessions.UpdateSessionGatewayBinding(token, gatewayUserID, gatewayAccessToken)
+}
+
 func (s *store) listSkills(tag, search string, limit, offset int) ([]skill, int) {
 	filtered := make([]skill, 0, len(s.skills))
 	for _, item := range s.skills {