Slim down CI runs per commit (#2911)

- Only run push-triggered workflows on main; PRs are covered by
  pull_request, so branch commits no longer run everything twice
- Test SQLite versions: single job, Python 3.13 + SQLite 3.25
- Fold coverage into the Python 3.14 test job (including serial
  tests) and remove test-coverage.yml; bump codecov-action to v5
- Cancel superseded in-progress runs on pull requests
- Prettier now also runs on pull requests

Claude-Session: https://claude.ai/code/session_012G7fa8HqJy9AkSjkocetAz

.github/workflows/playwright.yml

@@ -2,9 +2,15 @@ name: Playwright
 
 on:
   push:
+    branches:
+    - main
   pull_request:
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 permissions:
   contents: read