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  hardcoded English titles superseded by i18n).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/legal/LegalLayout.tsx

@@ -4,7 +4,7 @@
 // with no react-query provider: the operator contact is fetched with a plain fetch below.
 import { useEffect, useState } from "react";
 
-export const LAST_UPDATED = "June 14, 2026";
+const LAST_UPDATED = "June 14, 2026";
 
 // Contact shown on the legal pages: the instance operator's configured admin email, from the
 // public /auth/config. One shared fetch across the page; null when the operator set none.
@@ -19,7 +19,7 @@ function fetchOperatorContact(): Promise<string | null> {
   return contactPromise;
 }
 
-export function useOperatorContact(): string | null {
+function useOperatorContact(): string | null {
   const [contact, setContact] = useState<string | null>(null);
   useEffect(() => {
     let alive = true;