Add community infrastructure: issue + PR templates, CoC

Gitea issue templates (.gitea/issue_template/):
- bug.yml: structured form requiring version, environment,
  what-happened, repro steps, expected, optional logs. Routes
  security reports to security@librete.ch instead of public
  issues.
- feature.yml: prompts for the underlying problem before the
  proposed solution, plus alternatives and out-of-scope.

Pull request template (.gitea/PULL_REQUEST_TEMPLATE.md):
checklist for tests, lint, manual exercise, docs, and changelog.
Asks for explicit reviewer notes so trade-offs surface in the
PR description rather than being lost in chat.

CODE_OF_CONDUCT.md: links to Contributor Covenant 2.1 verbatim
rather than inlining; documents scope, reporting address
(conduct@librete.ch), and points enforcement at the Covenant's
own Enforcement Guidelines.

README links the docs/ tree, CONTRIBUTING, and the CoC so new
contributors find the entry points.

Closes #31.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,29 @@
+## Summary
+
+What does this PR change, and why?
+
+## Linked issue
+
+Closes #
+
+## Changes
+
+- 
+
+## Testing
+
+- [ ] `make test` passes locally
+- [ ] `make lint` passes locally
+- [ ] Added or updated tests for the changed behaviour
+- [ ] Manually exercised the affected flow (frontend / API / etc.)
+
+## Documentation
+
+- [ ] Updated relevant docs in `docs/`
+- [ ] Updated `CHANGELOG.md` if user-visible
+- [ ] No documentation change needed
+
+## Notes for reviewers
+
+Anything you want a reviewer to look at specifically? Trade-offs
+considered? Migrations to flag?