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/repository.go

@@ -7,6 +7,7 @@ type SessionRepository interface {
 	GetSession(token string) (session, bool, error)
 	DeleteSession(token string) error
 	RotateSession(oldToken string, ttl time.Duration) (session, bool, error)
+	UpdateSessionGatewayBinding(token string, gatewayUserID int, gatewayAccessToken string) error
 }
 
 type JobRepository interface {