Extract thumbnail path helpers into internal/thumb

The scanner (thumbnailForVideo, thumbnailForImage), the service importer
(generateThumbnail), and writeService.RegenerateThumbnail all reinvented
the same ".thumbnails/<base>.jpg" layout inline. Centralise the
convention in internal/thumb/path.go (ThumbnailDir, ThumbnailNameFor,
ThumbnailPathFor + DirName) so changing the on-disk layout is a one-line
change and the three callers cannot drift apart. Pure path math, no I/O —
callers retain their existing MkdirAll mechanism (scanner FS, os.MkdirAll
in service) for testability.

Covered by table-driven unit tests for trailing slashes, missing
extensions, dotfiles, and multi-dot filenames.

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