fix temporal bug

[mattmueller-stripe]

Problem

POST /webhooks/{pipeline_id} signaled the Temporal workflow with stripe_event, but
pipelineWorkflow only handles source_input.

Temporal drops signals with no matching handler, so
every signed webhook got HTTP 200 (Stripe never retried) and then silently vanished — never reaching the
pipeline.

Broken since #251 (2026-04-06) removed the stripe_event handler without updating this
route.

Fix

apps/service/src/api/app.ts: signal source_input with the SourceInputMessage envelope the workflow
actually expects, and return a 502 if the signal fails (instead of swallowing the error) so Stripe
retries.

Added tests covering valid signature → correct signal, invalid signature → 401/no signal, signal failure
→ non-2xx. Fixed a stale test title in workflow.test.ts that still said stripe_event.

Test plan

• pnpm test
• pnpm lint
• pnpm build