chore: repository hygiene — P2 cleanup

- .gitignore: add coverage/ and data/ patterns
- .dockerignore: exclude screenshots, tests, scripts, .github, docs
  assets from build context for faster Docker builds
- Delete docs/social-preview.html (one-time generator, no longer needed)
- Delete public/locales/.gitkeep (directory has de.json and en.json)
- scripts/seed-demo.js: replace hardcoded absolute path with portable
  resolve(__dirname, '..', 'data', 'oikos.db') default
- Add .github/PULL_REQUEST_TEMPLATE.md with summary, changes, checklist

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+## Summary
+
+<!-- What does this PR do? Link the related issue: Closes #123 -->
+
+## Changes
+
+- 
+
+## Checklist
+
+- [ ] `npm test` passes
+- [ ] Follows [CONTRIBUTING.md](../CONTRIBUTING.md) conventions
+- [ ] No new frontend dependencies (vanilla JS, no frameworks)
+- [ ] UI strings use `t('key')` (no hardcoded text)
+- [ ] CHANGELOG.md updated (if user-facing change)