ci: reuse backend dist/ in E2E instead of rebuilding (#488)

The backend CI job already runs `tsc` against backend/src/ and only
marks the job green after build + tests + lint + audit all pass. The
E2E job then checked out the same source and ran the same tsc again
inside the start-app composite.

Upload backend/dist/ as a 1-day retention artifact at the end of the
backend job, download it in the E2E job before invoking the composite,
and gate the composite's "Build backend" step on a new
`skip-backend-build` input. The update-screenshots job (which also
uses the composite but has no upstream backend job) leaves the input
at its default of 'false' and continues to build from source.

Savings are modest (~10-15s per E2E run, on top of what setup-node's
~/.npm cache already buys us) but the real win is the contract:
the dist/ that E2E exercises is now byte-for-byte the same artifact
that the backend job verified, eliminating a whole class of "works
in backend job, breaks in E2E" drift.

.github/actions/start-app/action.yml

@@ -14,6 +14,15 @@ inputs:
   port:
     required: false
     default: '3000'
+  skip-backend-build:
+    description: >
+      When 'true', skip running `npm run build` in backend/ and assume the
+      caller has already materialized backend/dist (e.g. via download-artifact
+      from an upstream job that already built and verified it). Leave 'false'
+      for standalone invocations like the update-screenshots job that do not
+      run in a job graph with a prior build step.
+    required: false
+    default: 'false'
 
 runs:
   using: composite
@@ -38,6 +47,7 @@ runs:
       working-directory: ./backend
 
     - name: Build backend
+      if: inputs.skip-backend-build != 'true'
       shell: bash
       run: npm run build
       working-directory: ./backend