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/issue_template/bug.yml

@@ -0,0 +1,60 @@
+name: Bug report
+about: Something is broken or doesn't work as documented
+labels: ["bug"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to file a bug. Please fill in
+        as much of the form as you can — the more concrete the
+        report, the faster we can fix it.
+
+        For security issues, **do not** open a public issue.
+        Email `security@librete.ch` instead.
+  - type: input
+    id: version
+    attributes:
+      label: librenotes version
+      description: Output of `librenotes version` or the Docker image tag.
+      placeholder: v0.1.0
+    validations:
+      required: true
+  - type: input
+    id: environment
+    attributes:
+      label: Environment
+      description: Browser + OS for frontend bugs; OS + Go version for backend bugs.
+      placeholder: Firefox 124 on macOS 14 / Go 1.22 on Debian 12
+    validations:
+      required: true
+  - type: textarea
+    id: what-happened
+    attributes:
+      label: What happened?
+      description: A clear, concrete description of the bug.
+    validations:
+      required: true
+  - type: textarea
+    id: reproduce
+    attributes:
+      label: Steps to reproduce
+      description: Numbered steps from a clean starting state.
+      placeholder: |
+        1. Sign in as a new user.
+        2. Create a note named "Test".
+        3. Click the toggle.
+        4. ...
+    validations:
+      required: true
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected behaviour
+    validations:
+      required: true
+  - type: textarea
+    id: logs
+    attributes:
+      label: Relevant logs / network traces
+      description: Any server logs, browser console errors, or HAR snippets.
+      render: shell