Dashboard detail, library filtering/pagination, simplified playlist times,

job descriptions

Dashboard: format breakdown (FLAC/MP3/etc. counts), approximate library
size (bitrate*length/8, same approximation `beet stats` itself uses --
verified to match its "100.5 GiB" output exactly against the real
library), and a credential auth-state summary per scope (configured y/n
plus, for Bandcamp, cookie expiry parsed locally from the stored cookie
jar -- deliberately not a live network probe like pipeline-status.sh's
Qobuz check, since this renders on every dashboard load).

Library: was one unfiltered page dumping all 4072 tracks. Added search
(artist/title/album), playlist and format filter dropdowns, and real
SQL-level pagination (LIMIT/OFFSET, not fetch-everything-then-slice).
beets_service gained count_items()/distinct_formats() to support this.

Playlists: cron_expr is still the stored/scheduled representation, but the
UI now shows and edits a plain daily time picker instead of raw cron
syntax -- every playlist schedule today is a simple daily HH:MM anyway.
playlist_service.cron_to_time()/time_to_cron() convert at the router
boundary; verified round-trip against all real playlist cron values.

Jobs: each maintenance job now shows a short one-line description of what
it actually does (MAINTENANCE_JOB_DESCRIPTIONS), and "next run" is
formatted consistently with playlists' time style (HH:MM, with a day
qualifier for non-today runs -- maintenance jobs can be weekly/monthly,
unlike playlists' plain daily schedule).

Verified end-to-end against the real production data (4072 tracks, 100.5
GiB, real credentials): stats/format-breakdown/search/pagination all
correct, all 7 credential scopes report configured with Bandcamp's real
cookie expiry (325 days), and a full authenticated page-render sweep
(dashboard, library with every filter combination, playlist detail,
jobs) all returned 200 with no template errors.

app/templates/playlists/detail.html

@@ -19,8 +19,9 @@
   <form method="post" action="/playlists/{{ playlist.id }}">
     <label class="checkbox-field"><input type="checkbox" name="active" value="true" {{ "checked" if playlist.active }}> Active</label>
     <div class="field">
-      <label for="cron_expr">Cron schedule</label>
-      <input type="text" id="cron_expr" name="cron_expr" value="{{ playlist.cron_expr or '' }}" placeholder="0 1 * * *">
+      <label for="sync_time">Sync time (daily)</label>
+      <input type="time" id="sync_time" name="sync_time" value="{{ sync_time }}">
+      <span class="hint">Leave blank for unscheduled/manual-only.</span>
     </div>
     <label class="checkbox-field"><input type="checkbox" name="no_m3u" value="true" {{ "checked" if playlist.no_m3u }}> Skip M3U</label>
     <div class="field wide">