docs: add backup notes and refresh checklist

deploy/BACKUP_RESTORE.md

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+# Backup And Restore
+
+The minimum persistent state for this project is:
+
+- `data/app.db`
+- `data/artwork/`
+- your music library mount, if the server machine is the primary storage location
+
+## What To Back Up
+
+Recommended:
+
+- entire `data/` directory
+- entire `media/` directory if the same host stores the original files
+- your `.env` or deployment environment settings
+
+Why:
+
+- `app.db` stores users, sessions, playlists, favorites, and scanned metadata
+- `artwork/` stores extracted embedded covers
+- `media/` contains the source files used to rebuild the library index
+
+## Simple Backup Example
+
+PowerShell:
+
+```powershell
+$stamp = Get-Date -Format 'yyyyMMdd-HHmmss'
+New-Item -ItemType Directory -Force -Path ".\\backups\\$stamp" | Out-Null
+Copy-Item -Recurse -Force .\\data ".\\backups\\$stamp\\data"
+Copy-Item -Recurse -Force .\\media ".\\backups\\$stamp\\media"
+```
+
+## Restore Example
+
+1. Stop the server.
+2. Restore `data/` from backup.
+3. Restore `media/` if needed.
+4. Start the server again.
+
+PowerShell:
+
+```powershell
+Copy-Item -Recurse -Force ".\\backups\\20260403-010000\\data\\*" ".\\data"
+Copy-Item -Recurse -Force ".\\backups\\20260403-010000\\media\\*" ".\\media"
+```
+
+## Notes
+
+- If `media/` is already backed up elsewhere, restoring `data/app.db` and `data/artwork/` is usually enough.
+- If `artwork/` is lost but `media/` is intact, the server can rebuild extracted covers during future scans.
+- If `app.db` is lost, the library can be rescanned from `media/`, but playlists, favorites, sessions, and users will be lost unless restored from backup.