● feat(#1008): library trash bin, admin bulk purge, auto-purge setting

Library files now move to a configurable-retention trash bin on delete
  instead of being hard-deleted from disk (default 30 days). Admins get a
  "Purge old" bulk action on the File Manager with a live preview, plus an
  optional auto-purge setting in Settings → File Manager that runs the same
  operation once per 24h when enabled (default off). Regular users see and
  manage their own trashed files; admins see everyone's. External (linked)
  files bypass trash since their bytes aren't under Bambuddy's control.

  - New `library:purge` permission (admin-only by default)
  - Nullable indexed `deleted_at` column on library_files; dialect-aware
    ALTER TABLE so the column actually gets added on PostgreSQL (raw
    DATETIME is SQLite-only syntax)
  - New `LibraryFile.active()` classmethod; every query site routed through
    it so trashed rows don't leak into listings, print dispatch, MakerWorld
    dedupe, or stats
  - Trash page: select-all + bulk restore/delete, per-row checkboxes, wider
    layout so datetime columns don't clip
  - Auto-purge: 24h throttle via `library_auto_purge_last_run` setting so
    the 15-minute sweeper cadence still runs the purge at most once per day
  - Save toast wired into every trash/auto-purge setting change
  - 17 new backend integration tests (service + routes + auto-purge throttle),
    8 new frontend tests, localised across all 8 UI languages
  - Wiki + website feature entries updated

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