Sources table: Media column (image coverage % + paywall marker)

Per Codex — make the table more decision-ready from data we already have.
Paywall is a domain-level hint, so it's a per-source flag (not a meaningful
rate): show image-coverage % plus a 🔒 marker for subscription domains in one
compact "Media" column (tooltip spells it out). source_health gains a
`paywalled` flag (is_paywalled on homepage/feed); also added to sources.csv.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/admin/+page.svelte

@@ -445,7 +445,7 @@
         <table class="srctable">
           <thead>
             <tr>
-              <th>Source</th><th class="num">Served</th><th class="num">Accept</th><th class="num">Dup</th>
+              <th>Source</th><th class="num">Served</th><th class="num">Accept</th><th class="num">Dup</th><th class="num">Media</th>
               <th>Last success</th><th>Next poll</th><th class="num">Fails</th><th>Status</th><th>Actions</th>
             </tr>
           </thead>
@@ -460,6 +460,9 @@
                 <td class="num">{s.served}</td>
                 <td class="num">{s.acceptance_rate != null ? s.acceptance_rate + '%' : '—'}</td>
                 <td class="num" title={s.accepted_dup_rate != null ? `${s.accepted_dup_rate}% of accepted were duplicates` : ''}>{s.duplicate_rate != null ? s.duplicate_rate + '%' : '—'}</td>
+                <td class="num media" title={`${s.image_coverage != null ? s.image_coverage + '% of served have an image' : 'no served articles yet'}${s.paywalled ? ' · subscription / paywalled domain' : ''}`}>
+                  {s.image_coverage != null ? s.image_coverage + '%' : '—'}{#if s.paywalled} <span class="pw">🔒</span>{/if}
+                </td>
                 <td class="dim">{s.last_success_at ? fwhen(s.last_success_at) : '—'}</td>
                 <td class="dim">{st === 'active' ? fwhen(s.next_due_at) : '—'}</td>
                 <td class="num">{s.failures || ''}</td>
@@ -791,6 +794,8 @@
   }
   .srctable td { padding: 8px 10px; border-bottom: 1px solid var(--line); vertical-align: baseline; }
   .srctable .num { text-align: right; font-variant-numeric: tabular-nums; }
+  .srctable .media { white-space: nowrap; }
+  .srctable .media .pw { font-size: 0.78rem; opacity: 0.75; }
   .srctable .dim { color: var(--muted); white-space: nowrap; font-size: 0.82rem; }
   .srctable .sname { font-weight: 600; color: var(--ink); }
   .srctable .sname .cat { display: block; font-weight: 400; font-size: 0.72rem; color: var(--muted); text-transform: capitalize; }