fix(importer): move single-file audiobooks into their folder on reorganize (#2894)

[zikeji]

Summary

Rename files gave a single-file audiobook its templated folder path as its own destination, so Author/Title.m4b was moved to a file named Author/Title (Year). proposedPathFor used AudiobookDestDir for every audiobook row, and moveTrackedFile then moved the file onto that path through the ebook branch.

A file row now resolves to <templated folder>/<its own name>, mirroring the single-file import branch:

• Joins the book's own ebook folder when that is the templated folder (added "(2)" to dir on import when ebook import dir is the same as the audiobook import dir (regression?) #2686), same as import.
• Otherwise uniquifies with the file's current folder treated as available. Before this, a file already sitting in Title (Year)/ was proposed as a move to Title (Year) (2), which is where the (2) folders in my library came from.
• The orphaned-sidecar cleanup and the sidecar write use file vs folder instead of format, so metadata.opf lands inside the folder.

Folder rows are unchanged.

PreviewImportDestination shares proposedPathFor, so the Fix Match preview for a single audiobook file now names <folder>/<file> instead of the folder. That's where the single-file import branch writes it, so the preview now matches the import. TestPreviewImportDestination_FormatHintWins is updated to expect the file inside the folder; it still asserts the format hint routes to the audiobook root.

Closes #2894

How it was verified

New tests in internal/importer/reorganize_test.go, failing on main (31ccc40):

--- FAIL: TestReorganize_SingleFileAudiobookMovesIntoFolder
    preview = [... Current:.../Jane Doe/My Book.m4b Proposed:.../Jane Doe/My Book (2020) Status:move ...], want move to ".../Jane Doe/My Book (2020)/My Book.m4b"
--- FAIL: TestReorganize_SingleFileAudiobookInPlaceIsNoop
    preview = [... Current:.../Jane Doe/My Book (2020)/My Book.m4b Proposed:.../Jane Doe/My Book (2020) (2) Status:move ...], want noop at ".../Jane Doe/My Book (2020)/My Book.m4b"
--- FAIL: TestReorganize_SingleFileAudiobookSidecarGoesInsideTheFolder
--- FAIL: TestReorganize_SingleFileAudiobookJoinsItsEbookFolder
    audiobook: {... Proposed:.../Jane Doe/My Book (2020) (2) Status:move ...}, want move to ".../Jane Doe/My Book (2020)/My Book.m4b"

All 18 TestReorganize_* pass on this branch.

End to end against an image built from this branch, using the repro from #2894 (flat Stephen King/The Shining.m4b, library scan, Rename files):

	v1.39.0	this branch
Proposed	Stephen King/The Shining (1977)	Stephen King/The Shining (1977)/The Shining.m4b
On disk after apply	file The Shining (1977), no extension	The Shining (1977)/The Shining.m4b
metadata.opf	not a directory warning	written inside the folder
Second preview	noop on the broken path	noop on the correct path

Follow-ups (not in this PR)

A file already flattened by the old behaviour has lost its extension, and this doesn't restore it. Recovery stays manual: rename it back into a folder, deregister the old path, rescan.

Checklist

• Every commit carries a Signed-off-by that matches its author (git commit -s), see Sign your work. Commits authored by a coding agent fail this check; author them as yourself.
• Changelog fragment added as changelog.d/<issue-number>-<slug>.md, not an edit to CHANGELOG.md, see changelog.d/README.md
• Tests added or updated
• docs/DEPLOYMENT.md updated if env vars, config, or upgrade path changed
• Wiki pages under docs/ updated if user-facing behaviour changed
• No new dependency, or its licence is permissive (Bindery is MIT; GPL, AGPL, SSPL and BUSL are not accepted) and THIRD_PARTY_LICENSES.md is regenerated

No docs change: the User Guide already says a reorganized library and a freshly imported one come out identical, and this makes that hold for single files.

Test plan

• gofmt, go vet ./..., golangci-lint run (v2.11.4), govulncheck ./... on Go 1.26.6
• go test ./cmd/... ./internal/...
• cd web && npm run build (no web/ changes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[vavallee]

Nice fix, the preview now matches what import does. One gap I hit: a file that's already been flattened (named Title (Year) with no extension) got proposed as a move into Title (Year) (2)/Title (Year), which just buries it. Pushed a small commit that reports that shape as an error with the recovery steps instead, plus a couple of tests and a credit line in the changelog. Holler if anything looks off.