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  hardcoded English titles superseded by i18n).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/lib/api.ts

@@ -78,7 +78,7 @@ export interface VideoDetail {
   duration_seconds: number | null;
 }
 
-export interface ChannelLink {
+interface ChannelLink {
   title: string | null;
   url: string;
 }
@@ -611,7 +611,7 @@ export interface SystemConfigData {
 }
 
 // Admin: result of testing the Plex server connection (also drives the library-picker).
-export interface PlexSection {
+interface PlexSection {
   key: string;
   title: string;
   type: string; // "movie" | "show"
@@ -656,13 +656,6 @@ export interface PlexCard {
   show_id?: string | null; // episode: the show's rating_key (for grouping a playlist by show/season)
   summary?: string | null; // episode
 }
-export interface PlexBrowseResult {
-  kind: "movie" | "show";
-  total: number;
-  offset: number;
-  limit: number;
-  items: PlexCard[];
-}
 // Unified cross-library browse (movies + shows mixed). `episodes` is populated only on a search that
 // also matches episodes (the grouped "Episodes" section).
 export interface PlexUnifiedResult {
@@ -823,7 +816,7 @@ export function plexFilterCount(f: PlexFilters): number {
 // Serialize the shared PlexFilters metadata fields onto a query string. Used by both the library grid
 // and the facets request so the two endpoints always agree on the filter set (paging/sort/sort_dir are
 // each caller's own concern and stay out of here).
-export function appendPlexFilters(u: URLSearchParams, f: PlexFilters): void {
+function appendPlexFilters(u: URLSearchParams, f: PlexFilters): void {
   if (f.genres?.length) u.set("genres", f.genres.join(","));
   if (f.genreMode === "all") u.set("genre_mode", "all");
   if (f.contentRatings?.length) u.set("content_ratings", f.contentRatings.join(","));
@@ -903,7 +896,7 @@ export interface DownloadProfile {
   sort_order: number;
 }
 
-export interface DownloadAsset {
+interface DownloadAsset {
   id: number;
   status: string;
   title: string | null;