fix(moves): organize moves into a subfolder of the source no longer fail as overlapping

[kevinheneveld]

The bug

Naming templates that append subfolders (narrator, edition, …) make organize request moves like:

/audiobooks/Stephen King/Elevation  →  /audiobooks/Stephen King/Elevation/Stephen King

— a target inside its own source. The copy-walk mover can't execute that shape (the recursive copy would descend into its own output, and the post-copy Directory.Delete(source, true) would delete the result), so the overlap guard failed the whole class with "Source and target paths overlap".

On my live library this hit 97 consecutive organize jobs — every book whose folder didn't yet contain the template's narrator subfolder.

The fix

Descend-shaped moves don't need to copy anything: two same-volume atomic renames do it safely —

1. rename the source aside (same parent, atomic): Elevation → Elevation.moving-<jobId>
2. recreate the vacated path: mkdir Elevation
3. rename into place: Elevation.moving-<jobId> → Elevation/Stephen King

with best-effort rollback if step 2/3 fails (the recreated skeleton is empty by construction, so it's dropped and the aside-rename is reversed). The inverse shape — flattening a folder up into its populated ancestor — stays blocked exactly as before.

Implementation notes:

• The rename logic lives in a MoveJobProcessor.DescendMove partial; the processor itself gains only the branch points (it was 1 line under the 500-line architecture rule).
• The existing completion path (history entry, notifications, scan enqueue, status) is shared unchanged.

Testing

• New integration tests drive ProcessJobAsync directly: the descend shape completes with files physically renamed into the child folder and a Moved history entry; the upward-overlap shape still fails with the original error and leaves files untouched.
• Full suite: 1191 passed.
• Live-verified on my instance before porting here: all 97 previously-failed jobs were requeued after this fix — 90 completed (the other 7 were stale requests whose source folders no longer exist).

Heads-up: #717 also works in the moves area — happy to rebase this on top of it if that lands first; the footprint here is deliberately small.

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