GetOrAddFolder was a check-then-insert with no unique constraint behind it,
so two callers racing the same folder could both miss the lookup and both
insert. Enforce identity in the schema and make the loser adopt the winner.
- LibraryFolder gains a SHA-256 PathHash (the MediaFile.Path/PathHash
precedent): Path is MySQL longtext, which cannot be indexed without a
prefix length and collates case-insensitively, so the unique index is on
(LibraryPathId, PathHash) instead.
- GetOrAddFolder and SetEtag catch a classified unique violation via the
existing TvContext.IsUniqueConstraintViolation seam (#308) and re-read.
- Dual-provider migration audits and collapses pre-existing duplicates
(repointing MediaFile, ParentId and ImageFolderDuration) before creating
the index; legacy rows keep a null hash and heal on the next scan.
- Tests: deterministic cross-connection race, 8x10 barrier stress with an
insert-attempt vacuity guard, classifier-inversion negative control, and
a real-migration dedupe test.
Refs #488#308fix#491
timothy
9a4f3e832d