feat: add contributing guidelines and pull request template; update schema comments and documentation

.github/PULL_REQUEST_TEMPLATE.md

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+## Summary
+
+<!-- What changed and why? -->
+
+## Change Type
+
+- [ ] Bug fix
+- [ ] Feature
+- [ ] Compatibility update
+- [ ] Documentation
+- [ ] Refactor
+
+## Cross-File Checklist
+
+- [ ] I read `CONTRIBUTING.md`.
+- [ ] Schema changes, if any, updated both runtime schema and `migrations/0001_init.sql`.
+- [ ] Persistent data changes, if any, updated backup export/import or documented why backup is not needed.
+- [ ] User-facing text changes, if any, updated all locale files.
+- [ ] Bitwarden client compatibility was considered for sync/API shape changes.
+- [ ] No secrets, tokens, private deployment values, or real vault data are included.
+
+## Checks
+
+- [ ] `npx tsc -p tsconfig.json --noEmit`
+- [ ] `npx tsc -p webapp/tsconfig.json --noEmit`
+- [ ] `npm run i18n:validate`
+- [ ] `npm run build`
+
+## Notes
+
+<!-- Anything reviewers should pay special attention to? -->