docs: add CODEOWNERS and issue/PR templates (#112)

Author: rubenhensen

.github/CODEOWNERS

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+# Code owners are automatically requested for review on matching PRs.
+# https://docs.github.com/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
+#
+# Default owner for everything. Adjust to a team (e.g. @encryption4all/<team>)
+# or add path-specific rules below as the maintainer set grows.
+* @rubenhensen

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a problem with PostGuard for Business
+title: ''
+labels: bug
+---
+
+## What happened
+
+<!-- A clear description of the bug. -->
+
+## Steps to reproduce
+
+1.
+2.
+3.
+
+## Expected behaviour
+
+<!-- What you expected to happen instead. -->
+
+## Environment
+
+- Version / commit / image tag:
+- Browser & OS (if relevant):
+
+## Additional context
+
+<!-- Logs, screenshots, or anything else useful. Do NOT paste secrets. -->
+
+<!--
+Found a security vulnerability? Do not file it here — report it privately.
+See SECURITY.md.
+-->

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+contact_links:
+  - name: Report a security vulnerability
+    url: https://github.com/encryption4all/postguard-business/security/advisories/new
+    about: Please report security issues privately, not as a public issue. See SECURITY.md.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea or improvement
+title: ''
+labels: enhancement
+---
+
+## Problem
+
+<!-- What are you trying to do, and what's getting in the way? -->
+
+## Proposed solution
+
+<!-- What you'd like to see happen. -->
+
+## Alternatives considered
+
+<!-- Other approaches you've thought about, if any. -->
+
+## Additional context
+
+<!-- Mockups, links, or related issues. -->

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+PR titles must follow Conventional Commits (enforced by the "Conventional Commit"
+check), e.g. `feat: ...`, `fix: ...`, `docs: ...`, `ci: ...`, `chore: ...`.
+-->
+
+## Summary
+
+<!-- What does this PR change, and why? -->
+
+## Related issue
+
+<!-- e.g. Closes #123 -->
+
+## Checklist
+
+- [ ] `npm run lint` and `npm run check` pass
+- [ ] Tests added/updated where it makes sense (`npm run test:unit` / `npm run test:e2e`)
+- [ ] Database changes ship a migration and pass `npm run db:check` (migration safety)
+- [ ] `.env.example` / docs updated if configuration or behaviour changed