feature request: Preserve media metadata across external drives (or NAS), avoid deletion on rescan when drive is disconnected

[nnotwen]

Checklist

• [x] I checked that this feature has not been requested before
• [x] I checked that this feature is not in the "Not planned" list
• [x] This feature will benefit the majority of users

Problem Description / Use Case

Some users (me included) keep their media libraries fragmented across multiple external drives due to large file sizes and limited storage capacity per device. Currently, if a rescan is triggered while one of these drives is disconnected, the app interprets the missing folders as deleted and removes the associated media entries from the database. This forces us to rematch metadata every time we swap a drive, making it cumbersome to manage libraries spread across several external drives.

I personally experienced this behavior with external drives, but I believe it may also affect NAS users. If a rescan occurs while a NAS share is offline (due to network downtime or server unavailability), the app may similarly purge metadata even though the media still exists.

Proposed Solution

• Persist media metadata by associating scanned folders with a stable storage identifier (e.g., device path, serial number, or other identifier accessible via Node/Electron libraries such as drivelist).
• During a rescan:
  • If a drive or NAS share is not present (disconnected or offline), retain the database entries for that storage.
  • Hide the associated metadata from the library view so users only see content from currently available drives/shares.
• Only delete entries if:
  1. The storage is present, and
  2. The folder path being tracked no longer exists on that storage.
• If possible, emulate how game libraries behave (e.g., Steam, Epic, Xbox): games appear and disappear seamlessly depending on which drives are connected and where the games are installed. This would make media entries dynamically visible only when their storage is available, without requiring rescans or risking metadata loss.
• Provide an option for users to manually remove a database associated with a drive or NAS share, so that unused or permanently disconnected storage doesn’t clutter the database.