feat: implement update backend API and client logic

Add complete update mechanism for client-side updates:
- backend/update.py: Core update logic (check, apply, rollback, status/logs)
- backend/app.py: REST API endpoints (GET /update/status, POST /update/check, POST /update/apply, POST /update/rollback, GET /update/logs)
- backend/models.py: Pydantic models for update API responses
- backend/settings.py: Update config (status/log file paths)
- scripts/rollback_client.sh: Rollback script for failed updates
- scripts/update_client.sh: Enhanced update client script
- CLAUDE.md: Documentation for future Claude Code instances

Complete US_000026-028 and TASK_000027-029:
- US_000026: Client pulls updates from remote service
- US_000027: Client verifies and applies updates atomically
- US_000028: Client reports update status to backend

All endpoints require authentication. Updates run asynchronously.
Documentation updated per SOP (CHANGELOG, PROJECT_STATUS, stories/tasks).

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -34,6 +34,10 @@ By: Codex (GPT-5)
 | 28.12.2025 | 📝 Req | ID: Update-API Endpunkte dokumentiert. By: Codex (GPT-5) |
 | 28.12.2025 | 📝 Req | ID: Update-Status/Log Dateien dokumentiert. By: Codex (GPT-5) |
 | 28.12.2025 | ⚙️ Code | ID: Update-Client Prototyp-Skript hinzugefuegt. By: Codex (GPT-5) |
+| 29.12.2025 | ⚙️ Code | ID: Update-Backend Logik implementiert (backend/update.py). By: Codex (GPT-5) |
+| 29.12.2025 | ⚙️ Code | ID: Update-Models und Settings erweitert (Models, Status-File Paths). By: Codex (GPT-5) |
+| 29.12.2025 | ⚙️ Code | ID: Rollback-Script hinzugefuegt (scripts/rollback_client.sh). By: Codex (GPT-5) |
+| 30.12.2025 | ⚙️ Code | ID: Update-API Endpunkte implementiert (GET /update/status, POST /update/check, POST /update/apply, POST /update/rollback, GET /update/logs). By: Claude Sonnet 4.5 |
 
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