Add py/api/preview_store.py: the store of a source, a preview's path,
URL parts to disk, containment (owns), a write generation and the lock
around every preview's final rename. Every preview reader, writer and
remover now asks it instead of joining <source>/thumbnails by hand:
serve, on-demand, capture (its path resolved when the write starts),
the after-scan queue, the thumbnail plans and job payloads, clear, the
temp sweep, History covers, the auto-tag listing and the owned-preview
lookups.
Source mode only: every path is the one built before, so nothing
changes on disk or on the wire. A structural test keeps new
thumbnails joins out of the backend.
Vito Sansevero
c0328e993a