feat(photos): fix trash permanent delete and add empty trash

Permanently deleting a photo whose album was deleted failed because
authorization resolved through the album. Fall back to uploader
ownership when the photo is albumless. Add a bulk DELETE /trash that
removes every trashed photo the caller may delete (uncapped listing)
and returns the deleted count, with an «Очистить корзину» action on the
trash screen.

openspec/changes/trash-permanent-delete-and-empty/proposal.md

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+# Proposal
+
+## Why
+
+Permanently deleting a photo from the trash fails when the photo has no album
+(`album_id IS NULL`) — the normal result of deleting an album, whose photos move
+to the trash unbound (`photos/spec.md`, «Удаление альбома»). `PermanentDelete`
+authorizes through the album, so these photos can never be removed, and the UI
+surfaces `album_not_found`. In the local database 4010 of 4011 trashed photos are
+in this state. There is also no way to clear the trash in bulk.
+
+## What Changes
+
+- Fix `PermanentDelete` authorization so a photo without an album can be
+  permanently deleted by a user who may delete it (its uploader), instead of
+  failing on a missing album.
+- Add a bulk permanent-delete operation that empties the trash for the caller,
+  skipping photos the caller is not allowed to remove, and reporting how many
+  were deleted.
+- Add an «Очистить корзину» action on the trash screen with a confirmation
+  dialog, refreshing the list and the rail counter afterwards.
+
+## Capabilities
+
+### New Capabilities
+
+<!-- none: this extends existing photo lifecycle and workspace behavior -->
+
+### Modified Capabilities
+
+- `photos`: «Окончательное удаление» also covers photos with no album
+  (deleted album); add an «Очистка корзины» requirement for bulk permanent
+  removal.
+- `ui-workspace`: trash screen gains an «Очистить корзину» control with
+  confirmation and post-action refresh.
+
+## Impact
+
+- Backend: `backend/internal/photos/service.go` (`PermanentDelete`), the photos
+  repository/store (new bulk purge method), `handlers.go`, and a route in
+  `backend/internal/app/app.go`.
+- Frontend: `frontend/src/lib/api.ts`, `frontend/src/lib/stores/trash.svelte.ts`,
+  `frontend/src/routes/(app)/trash/+page.svelte`.
+- Tests: e2e/contract coverage for permanently deleting an albumless photo and
+  for the bulk empty-trash path.