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

.github/workflows/ci.yml

@@ -68,6 +68,17 @@ jobs:
         working-directory: ./backend
         run: npm audit --audit-level=high
 
+      # Hand the freshly-verified dist/ to the E2E job so it does not have to
+      # re-run `tsc` on the same source. retention-days is the minimum; this
+      # artifact is only needed for the downstream e2e job in the same run.
+      - name: Upload backend dist
+        uses: actions/upload-artifact@v7
+        with:
+          name: backend-dist
+          path: backend/dist
+          retention-days: 1
+          if-no-files-found: error
+
   frontend:
     name: Frontend (Build, Lint)
     runs-on: ubuntu-latest
@@ -158,8 +169,20 @@ jobs:
       - name: Checkout Code
         uses: actions/checkout@v6
 
+      # Reuse the dist/ that the `backend` job produced and verified, instead
+      # of running `tsc` a second time against the same source tree. The
+      # composite action's `skip-backend-build` input short-circuits its build
+      # step when this artifact is already in place.
+      - name: Download backend dist
+        uses: actions/download-artifact@v7
+        with:
+          name: backend-dist
+          path: backend/dist
+
       - name: Start app & install Playwright
         uses: ./.github/actions/start-app
+        with:
+          skip-backend-build: 'true'
 
       - name: Run E2E tests
         run: npx playwright test