docs: add BackupController task record

docs/Tasktracking/2026-04-26-add-backup-controller.md

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+# Task Record
+
+Date: 2026-04-26
+Related Module: web/controller
+Change Type: Add
+
+## Background
+Need BackupController to expose backup/restore API endpoints for the web panel. This is part of the backup feature implementation.
+
+## Changes
+- Created `web/controller/backup.go` with BackupController struct embedding BaseController
+- Added 5 API endpoints: createBackup, restoreBackup, deleteBackup, listBackups, downloadBackup
+- Uses service.BackupService for business logic
+
+## Impact
+- New file only, no existing files modified
+- Routes will be wired in a later task
+
+## Verification
+- `go build ./...` passed with no errors
+- `gofmt -l -w .` produced no changes (already formatted)
+
+## Risks And Follow-Up
+- Routes not yet wired to gin router (Task 5)
+- BackupService must be initialized before controller wiring