docs(release): Beta channel procedure: branch, version, changelog, tag, promote, roll back #1496

Part of the Beta channel epic #1499.

Problem

The release process documents only stable releases. The beta path has several traps that are easy to miss:

• scripts/extract-changelog-section.sh needs a section ## [0.5.0-beta.1] (the Publish GitHub Release job fails otherwise);
• .github/workflows/version-bump.yml runs when a PR into main is closed and drops any -prerelease from pubspec.yaml, so a beta version only exists on the release branch and the tag has to be set by hand;
• the tag and the pubspec semver must match (version-hint warns only);
• the CHANGELOG of a beta is a delta, the final release repeats everything.

Scope

• docs/tags-and-releases.md: a Beta channel section: branch release/X.Y.Z, version: X.Y.Z-beta.N+B in pubspec, ## [X.Y.Z-beta.N] in CHANGELOG, the annotated tag, what the workflow does, what Stable users see (nothing) and Beta users see (an update).
• Promote: pubspec X.Y.Z, ## [X.Y.Z] with the full notes, tag X.Y.Z (the beta tags stay).
• Roll back: mark the bad stable release as pre-release (or delete it) so releases/latest returns the previous one; the AUR / APT / DNF side needs its own revert (describe).
• docs/release-checklist.md: a beta checklist (smoke subset, how long a beta must be out, blockers) and the promote step; update the Versioning part that now says 0.4.x.
• CONTRIBUTING.md: link.
• Russian summary in docs/tags-and-releases.md follows the file's language.

Acceptance

• Following the document step by step produces a beta pre-release and then a stable one on a fork.
• The traps above are each named with the symptom and the fix.