docs: prominent migration banner at the top of README (#669)

GitHub-flavored `> [!IMPORTANT]` callout right below the title, before
the badges/hero block, so it's the first thing a visitor or repo browser
sees in the rendered README. Mirrors the in-app banner (#687) so an
operator gets the same message whether they're browsing the repo or
logged into the admin dashboard.

Body covers:
- Image-path change with the literal new path
- Branch rename (beta → main, main → stable) with auto-redirect note
- Link to docs/migration-to-org.md for the exact compose-file edit

Remove (or downgrade to a regular note) after the migration window
settles, same lifecycle as the in-app banner constant.

README.md

@@ -1,5 +1,13 @@
 # 📸 PicPeak - Open Source Photo Sharing for Events
 
+> [!IMPORTANT]
+> **PicPeak has moved to its own GitHub organization.**
+>
+> - **Docker images** are now published at `ghcr.io/picpeak/picpeak/{backend,frontend}`. The old path (`ghcr.io/the-luap/picpeak/...`) is no longer served — update your `docker-compose.yml`.
+> - **Branches**: active development is now on `main` (was `beta`); the curated stable channel is now `stable` (was `main`). Existing PRs and clones auto-redirect via GitHub.
+>
+> See **[`docs/migration-to-org.md`](docs/migration-to-org.md)** for the one-line `docker-compose.yml` edit and full details.
+
 <div align="center">
   <img src="docs/picpeak-logo.png" alt="PicPeak Logo" width="300" />