feat: watch a NextExplorer location as the consume folder

[helix-a]

Summary

Closes #10. Set Consume Folder gets a third source: a NextExplorer location (default Paperless). Documents dropped into it are imported and then deleted, the same as with FileBrowser Quantum (#9).

• Mount: NextExplorer's data volume, read-write, at /mnt/nextexplorer, only while this source is selected. PAPERLESS_CONSUMPTION_DIR is /mnt/nextexplorer/<location>. Read-write is required for the reason feat: watch a FileBrowser Quantum folder as the consume folder #9 gives. NextExplorer chowns its volume to uid 1000 on every start, the same uid Paperless runs as, so no idmap is needed.
• The location is NextExplorer's own. The action runs NextExplorer's add-location directly (feat: let dependents run add-location Start9Labs/nextexplorer-startos#6 opens it with access: 'dependent'). NextExplorer's own checks apply to the name, so it cannot land in _users or on a reserved prefix. An existing location is accepted, and the location shows up under Locations in NextExplorer. Nextcloud already exposes every NextExplorer location except _users as external storage, so drop-to-ingest from Nextcloud (Feature Request: Optionally share Paperless consume folder with Nextcloud for drop-to-ingest #3) works.
• Ordering: the OS lets only a declared dependent run the action, so the handler does three things in order:
  1. Refuses up front if NextExplorer isn't installed.
  2. Declares the nextexplorer dependency and runs add-location.
  3. Writes the store only after that succeeds. So a rejected name never reaches the daemon; on failure the dependency set is recomputed from the unchanged store.
• Dependency: nextexplorer, exists, >=3.1.0:1 && <4.0.0:0, declared only while it is the source. This is the same upper cap Nextcloud uses.
• paperlessDaemons takes one consumeFolder argument (filebrowser | nextexplorer | null) in place of filebrowserSubfolder. The install bootstrap still passes null, so feat: watch a FileBrowser Quantum folder as the consume folder #9's fix for the re-running init is kept.
• The prefill now reads with .once(), per the guide.
• nextexplorer-startos#next is added as a type-only dependency, for the typed mountDependency.
• The version goes to 2.20.15:6, since :5 is on community-prod.

Draft until the SDK 3.0 sweep lands here

Running another package's action with input is @start9labs/start-sdk 3.0.0 (Start9Labs/start-technologies#4069), which needs StartOS 0.4.0.2. The 3.0.0 bump belongs to the fleet sweep's PR, so against 2.0.9 this does not type-check. Against a local 3.0.0 build, startos/ is clean. The only error is inside node_modules/nextexplorer-startos (the sweep's §5 execFail change), which clears once NextExplorer's own bump lands on its next.

To finish:

• merge feat: let dependents run add-location Start9Labs/nextexplorer-startos#6 first
• rebase onto this repo's sweep PR
• match the nextexplorerVersionRange floor to the NextExplorer version that ships chore: bump start-sdk to 2.0.7 (2.20.15:3) #6
• get the build gate green
• verify on a 0.4.0.2 box:
  • the location appears in NextExplorer
  • a file dropped from NextExplorer's UI (and from Nextcloud) is imported and removed, i.e. inotify fires across the mount
  • a reserved name is rejected with the previous choice kept
  • switching back to the private folder drops the mount and the dependency

Not exercised on a box.

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