fix(backup): orthogonal code review fixes + UI/UX overhaul

## Critical fixes
- H1: Fix env_id/env/environment_id triple inconsistency in API route
  (_action_routes.py now passes 'environment_id', matches scheduler)
- H2: Decompose BackupPlugin.execute() — CC 13 → 5 methods, CC ≤ 5 each
- H3: Fix unhandled int() ValueError on non-numeric dashboard_ids
- H4: Add concurrent guard test with API-style params (env key fallback)
- H5: Make RetentionPolicy configurable via StorageConfig
  (retention_daily/weekly/monthly in config model + backend plugin)
- Fix: storage DELETE route missing 'await' on async delete_file (bug)

## UI/UX overhaul
- NEW: centralized cron utility (frontend/src/lib/utils/cron.ts)
  - validateCron(), calcNextCronRun(), formatNextRun()
  - Used by BackupManager, BackupDashboardModal
- BackupManager: replace custom BackupList with shared FileList
  - Adds: download, delete, bulk actions, search, sort, pagination
  - Adds: cron next-run preview below schedule input
  - Adds: auto-open TaskDrawer on backup task creation
- BackupDashboardModal: dead 'Cron Help' button removed
  - Adds: live cron validation + next-run preview
- StorageSettings: adds retention daily/weekly/monthly fields
- BackupCreateRequest type fixed to match actual API contract
- i18n: 7 new keys for en/ru (next_run, retention_*)

frontend/src/lib/i18n/locales/en/tasks.json

@@ -18,6 +18,7 @@
   "schedule_enabled": "Enabled",
   "cron_label": "Cron Expression",
   "cron_hint": "e.g., 0 0 * * * for daily at midnight",
+  "next_run": "Next run",
   "footer_text": "Task continues running in background",
   "drawer": "Task drawer",
   "close_drawer": "Close drawer",