Share repository read methods (Get, GetAll, Read, ReadAll, Exists, Count,
CountAll) did not apply an owner filter, so non-admin users saw shares
belonging to other users. The write paths already enforced per-user ownership;
this brings reads in line with them.
Add an addRestriction()/ownerFilter() based scope to share reads, keeping
admins and the headless public-share resolution path unrestricted. Route share
and player Delete through a new base-repo deleteOwned() primitive that applies
the ownership predicate in the DELETE's WHERE clause (atomic, no select-then-
delete window) and classifies a zero-row result as permission-denied vs
not-found, mirroring updateOwned. The addRestriction helper and the write-miss
classifier are hoisted onto the base repository so player and share share one
implementation.
Also map rest.ErrPermissionDenied and rest.ErrNotFound in the Subsonic error
handler so ownership/not-found failures from the rest-backed repositories
return the proper Subsonic codes (50 / 70) instead of a generic error.
Covered by unit tests (persistence, subsonic error mapping) and an end-to-end
cross-user sharing isolation test.
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