Cache Playwright browsers in CI

Playwright browser download (25s) is now cached by pnpm-lock hash.
On cache hit, only install system deps (fast). On miss, full install.

Current E2E timings (first run):
- Postgres container: 19s
- BRouter JAR: cached
- Berlin segment: 6s (will be cached next run)
- Playwright install: 25s → cached after first run
- BRouter startup: 9s
- Tests: 27s

Expected after caching: ~60s total (down from ~107s).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -139,7 +139,20 @@ jobs:
         env:
           BROUTER_URL: http://localhost:17777
 
-      - run: pnpm exec playwright install --with-deps chromium
+      - name: Cache Playwright browsers
+        id: playwright-cache
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/ms-playwright
+          key: playwright-${{ hashFiles('pnpm-lock.yaml') }}
+
+      - name: Install Playwright
+        if: steps.playwright-cache.outputs.cache-hit != 'true'
+        run: pnpm exec playwright install --with-deps chromium
+
+      - name: Install Playwright deps only
+        if: steps.playwright-cache.outputs.cache-hit == 'true'
+        run: pnpm exec playwright install-deps chromium
 
       - name: Run E2E tests
         run: pnpm test:e2e