docs(changelog): date the 4.0.0 section

The 4.0.0 tag and the GitHub release exist since 2026-09-10, but the changelog
still showed the section as `[Unreleased] - 4.0.0`. Rename the heading to
`## [4.0.0] - 2026-09-10`, and delete the release TODO comment that the release
answers.

The date comes from the GitHub release, and not from the local clock. The
release is the single source of truth for the version date.

Rejected: an empty `[Unreleased]` section above 4.0.0. The next change adds it,
and an empty section adds no information now.

Also fixed the Docs workflow deployment. The `github-pages` environment allowed
only the `main` branch, so the deploy job for tag `4.0.0` failed with
"Tag 4.0.0 is not allowed to deploy to github-pages". The fix is a repository
setting, and not a file: a `*.*.*` tag deployment policy now matches the
workflow's tag trigger.

Rejected: a trigger on `push` to `main`. The reference must show the release
version, and only the tag gives it.

Verified: re-ran run 34449792452 on GitHub. Both jobs pass, and
https://bunnyway.github.io/bunny-stream-android/ serves the site.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013TAtJoLwS3VW6bHDfom9T7

Author: bogdan-at-bunny

CHANGELOG.md

@@ -7,7 +7,7 @@ All notable changes to Bunny Stream Android are documented in this file. The for
 - `MINOR` versions add functionality in a backward-compatible way.
 - `PATCH` versions include backward-compatible bug fixes and maintenance updates.
 
-## [Unreleased] - 4.0.0
+## [4.0.0] - 2026-09-10
 
 The live streaming release, bundled with an architecture refactor. Existing integrations: see
 [MIGRATING.md](MIGRATING.md).
@@ -226,9 +226,6 @@ see [MIGRATING.md](MIGRATING.md) for before/after examples.
   persistable URI permission) and call `continueUpload` on the next launch, or run the upload from
   a foreground service or `WorkManager` job.
 
-<!-- TODO before the 4.0.0 release: check the "Changed" bullets against the final API and
-     move the entries to a dated 4.0.0 section. -->
-
 ## [3.3.1] - 2026-08-21
 
 ### Fixed