chore: Phase 1 hygiene — drop unused imports/exports/types (behavior-neutral)

Machine-baseline harvest (ruff + knip), all tsc/parse-green, no runtime change:
- backend: remove 3 unused imports (channels/playlists/youtube) via ruff; drop the
  unused `job` binding in unshare_download (keep the _own_job ownership guard call).
- frontend: remove `export` from 23 internally-used-only symbols (knip "unused
  exports") to shrink the public surface; delete 2 genuinely-dead declarations
  (PlexBrowseResult — leftover from the removed /browse route; WIDGET_TITLES —
  hardcoded English titles superseded by i18n).

Held back for a decision (unused here = possibly-unwired, NOT dead — flagged, not
removed): e2ee.lock()/clearDevice() (security primitives never wired to logout / a
"forget device" feature) and loadDefaultViewFilters (App reimplements it inline — a
DRY issue). See siftlode-ops/CODE-HYGIENE.md.

frontend/src/components/DataTable.tsx

@@ -9,7 +9,7 @@ import { useDismiss } from "../lib/useDismiss";
 // other modules (e.g. the playlist manager) can reuse it. Below `md` it falls back to a card
 // list built from the same column definitions.
 
-export type ColumnFilter<T> =
+type ColumnFilter<T> =
   | { kind: "text"; get: (row: T) => string }
   | { kind: "select"; options: { value: string; label: string }[]; test: (row: T, value: string) => boolean }
   | { kind: "multi"; options: { value: string; label: string }[]; test: (row: T, values: string[]) => boolean };