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>

.gitignore

@@ -17,9 +17,13 @@ logs/
 .DS_Store
 Thumbs.db
 
-# Build-Artefakte
+# Build-Artefakte & Test-Coverage
 dist/
 .cache/
+coverage/
+
+# Lokales Docker-Volume
+data/
 
 # IDE
 .vscode/