fix: compare updates via release tag (#745)

## ℹ️ Description
- Link to the related issue(s): Issue #N/A
- Describe the motivation and context for this change.
Ensure update-check compares against release tags instead of moving
branch tips and keep tests/translations in sync.

## 📋 Changes Summary
- compare release commit via tag name first and fall back only when
missing
- update update-checker tests for commit-ish resolution and tag-based
release data
- refresh German translations for update-checker log strings

### ⚙️ Type of Change
Select the type(s) of change(s) included in this pull request:
- [x] 🐞 Bug fix (non-breaking change which fixes an issue)


## ✅ Checklist
Before requesting a review, confirm the following:
- [x] I have reviewed my changes to ensure they meet the project's
standards.
- [x] I have tested my changes and ensured that all tests pass (`pdm run
test`).
- [x] I have formatted the code (`pdm run format`).
- [x] I have verified that linting passes (`pdm run lint`).
- [x] I have updated documentation where necessary.

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* More reliable update checks by resolving commits from tags, branches
or hashes and robustly comparing short vs full hashes.
* Improved prerelease handling to avoid inappropriate preview updates
and better handling of missing release data.

* **Localization & UX**
* Error and prerelease messages now use localized strings; commit dates
shown consistently in UTC and short-hash form.

* **Tests**
* Updated tests to cover the new resolution flow, error cases, and
logging behavior.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/kleinanzeigen_bot/resources/translations.de.yaml

@@ -520,10 +520,8 @@ kleinanzeigen_bot/utils/web_scraping_mixin.py:
 #################################################
 kleinanzeigen_bot/update_checker.py:
 #################################################
-  _get_commit_date:
-    "Could not get commit date: %s": "Konnte Commit-Datum nicht ermitteln: %s"
-  _get_release_commit:
-    "Could not get release commit: %s": "Konnte Release-Commit nicht ermitteln: %s"
+  _resolve_commitish:
+    "Could not resolve commit '%s': %s": "Konnte Commit '%s' nicht aufloesen: %s"
   check_for_updates:
     "A new version is available: %s from %s UTC (current: %s from %s UTC, channel: %s)": "Eine neue Version ist verfügbar: %s vom %s UTC (aktuell: %s vom %s UTC, Kanal: %s)"
     "Could not determine commit dates for comparison.": "Konnte Commit-Daten für den Vergleich nicht ermitteln."
@@ -531,8 +529,6 @@ kleinanzeigen_bot/update_checker.py:
     "Could not determine local version.": "Konnte lokale Version nicht ermitteln."
     "Could not determine release commit hash.": "Konnte Release-Commit-Hash nicht ermitteln."
     "Could not get releases: %s": "Konnte Releases nicht abrufen: %s"
-    "Failed to get commit dates: %s": "Fehler beim Abrufen der Commit-Daten: %s"
-    "Failed to get release commit: %s": "Fehler beim Abrufen des Release-Commits: %s"
     ? "Release notes:\n%s"
     : "Release-Notizen:\n%s"
     "You are on the latest version: %s (compared to %s in channel %s)": "Sie verwenden die neueste Version: %s (verglichen mit %s im Kanal %s)"