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>

backend/settings.py

@@ -45,6 +45,12 @@ class Settings:
         self.update_status_url: str = os.getenv(
             "SKD_UPDATE_STATUS_URL", "https://update.wlkns.org/status"
         )
+        self.update_status_file: str = os.getenv(
+            "SKD_UPDATE_STATUS_FILE", "/var/lib/skd/update_status.json"
+        )
+        self.update_log_file: str = os.getenv(
+            "SKD_UPDATE_LOG_FILE", "/var/lib/skd/update_logs.jsonl"
+        )
         # Paths/tools
         self.notify_send_path: str = os.getenv("SKD_NOTIFY_SEND_PATH", "notify-send")
         self.sound_player: str = os.getenv("SKD_SOUND_PLAYER", "paplay")