docs: align v0.4.0 release materials with final commit count and add RELEASE_NOTES.md

The v0.4.0 overview line in CHANGELOG.md and the opening paragraph of
the v0.4.0 blog post both cited '167 commits' — a stale count from an
earlier mid-release snapshot, before the final rounds of merges and
the frontend hardening sprint landed. The actual delta between v0.3.3
and v0.4.0 is 281 commits (git rev-list --count v0.3.3..v0.4.0).

Changes:

* CHANGELOG.md line 12: '167 commits' → '281 commits', and added an
  explicit mention of the frontend security hardening sprint to the
  overview summary so the release scope accurately reflects what
  shipped.

* docs-site/blog/2026-02-07-v0.4.0-release.md line 8: '167 commits'
  → '281 commits', added 'and WhisperX' to the GPU upstream patch
  credit (the 273x speaker assignment speedup went upstream to
  WhisperX, not just PyAnnote), and added the frontend hardening
  sprint to the 'what this release became' summary.

* RELEASE_NOTES.md (new): Comprehensive GitHub release notes at the
  repo root, referenced by docs/RELEASE_PLAN_v0.4.0.md Step 4.
  Mirrors the full CHANGELOG v0.4.0 section reorganized as
  release-note prose with 18 top-level sections (Enterprise Auth,
  Native Pipeline, PyAnnote v4 & Speaker Intelligence, Hybrid Neural
  Search, Cloud ASR, Groups & Sharing, Upload & Media, Frontend
  Hardening, UX Polish, PWA & Mobile, Infrastructure, Security, i18n,
  How to Update, Breaking Changes, Bug Fixes, Contributors, Full
  Details). Includes an embedded publish command + preconditions
  checklist in HTML comments (invisible to GitHub's markdown
  renderer) for the release publisher. This file is a temporary
  release artifact — archive or delete after publish.

Verified docs-site builds cleanly with the updated content.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: davidamacey

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Overview
 
-Major release combining enterprise-grade authentication, native transcription pipeline, neural search, GPU optimizations, cloud ASR providers, comprehensive speaker intelligence, Progressive Web App support, and dozens of features built from processing 1,400+ real-world recordings over two months of development (167 commits). This release significantly improves security, performance, search capabilities, and mobile usability.
+Major release combining enterprise-grade authentication, native transcription pipeline, neural search, GPU optimizations, cloud ASR providers, comprehensive speaker intelligence, Progressive Web App support, a frontend security hardening sprint, and dozens of features built from processing 1,400+ real-world recordings over two months of development (281 commits). This release significantly improves security, performance, search capabilities, and mobile usability.
 
 ### Added