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/models.py

@@ -34,3 +34,32 @@ class LoginRequest(BaseModel):
 class LoginResponse(BaseModel):
     token: str
     expires_in: int
+
+
+class UpdateStatus(BaseModel):
+    current_version: str
+    last_status: str
+    last_error: Optional[str] = None
+    last_timestamp: Optional[str] = None
+
+
+class UpdateCheckResponse(BaseModel):
+    available: bool
+    latest_version: str
+    artifact_url: str
+    sha256: str
+    message: Optional[str] = None
+
+
+class UpdateActionResponse(BaseModel):
+    started: bool
+    message: str
+
+
+class UpdateLogEntry(BaseModel):
+    timestamp: str
+    status: str
+    message: Optional[str] = None
+    version: Optional[str] = None
+    error: Optional[str] = None
+    device_id: Optional[str] = None