Add Docker update support via Watchtower integration (#1330)

* Add Docker update support via Watchtower integration

Add web-based Docker container updates using Watchtower HTTP API.
When configured with WATCHTOWER_API_URL and WATCHTOWER_API_TOKEN
environment variables, administrators can trigger container updates
from the Update Manager page.

Features:
- WatchtowerClient service for Watchtower HTTP API communication
- Docker update progress page with animated Docker whale logo
- Real-time step tracking: Trigger, Pull, Stop, Restart, Health Check, Verify
- CSP-compatible progress bar using CSS classes
- Translated UI strings via Stimulus values
- Health endpoint polling to detect container restart
- Watchtower setup documentation for Docker installations
- WatchtowerClient made nullable for non-Docker installations
- Unit tests for WatchtowerClient

* Fixed translation message IDs

* Switch Watchtower docs to maintained nicholas-fedor fork

The original containrrr/watchtower is no longer maintained (last release
Nov 2023). Point users to the drop-in compatible active fork and add an
info note explaining why. No code changes — the HTTP API is identical,
so WatchtowerClient works against either image.

* Fixed exception when github is not reachable

* Only show version string in health endpoint, when user has permissions

* Do not expose watchtower API port in example docker-compose file

* Show if updates, backup restore and backup download are allowed in update manager page

* Report 'not authorized' for version in health endpoint if user lacks permission

---------

Co-authored-by: Jan Böhmer <mail@jan-boehmer.de>

.env

@@ -76,6 +76,12 @@ DISABLE_BACKUP_RESTORE=1
 # When enabled, users must confirm their password before downloading.
 DISABLE_BACKUP_DOWNLOAD=1
 
+# Watchtower integration for Docker-based updates.
+# Set these to enable one-click updates via the Update Manager UI.
+# See https://containrrr.dev/watchtower/ for Watchtower setup.
+WATCHTOWER_API_URL=
+WATCHTOWER_API_TOKEN=
+
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