Edit issue and change descriptions in the browser

Why

The browser could file, discuss, triage, close, and reopen an issue or a change, but it could not correct what one of them said. An issue needed vole issue edit, and a change request’s title or description could not be corrected after it was opened on either surface. Closes #180.

What changed

• Added GET/POST /r/{name}/issues/{id}/edit and the matching change-request route, each with its own page.
• Both pages are offered to the item’s author and to a maintainer. The GET form and the POST handler ask the same capability, and a signed-in stranger gets a 403.
• Issue edits use the existing metadata.UpdateIssue. Change edits use a new metadata.SetRequestDetails, which validates the whole record and is not limited to an open request because the prose is not what an approval names.
• The writes are audited as web.issue.edit and web.request.edit, and a change edit re-checks the request tree like the other change writes.
• The item page gained an Edit action. Discussion, labels, assignees, checks, and approvals are untouched.
• Documented the browser edit in the issue and change-request docs.

Verification

• go test -tags libsqlite3,sqlite_fts5 ./metadata ./server/web
• go test -tags libsqlite3,sqlite_fts5 ./server -run '^(TestIssueCanBeClosedAndReopenedInTheBrowser|TestChangeCanBeDiscussedAndClosedInTheBrowser|TestNewIssueAndChangePages|TestIssueAndChangeDetailsCanBeEditedInTheBrowser)$'
• go vet -tags libsqlite3,sqlite_fts5 ./metadata ./server/web ./server
• vole workflow check
• vole workflow run verify