feat(web): per-Series detail page keyed by site:series_id (#144)

backend/internal/web/templates/series-detail.html

@@ -0,0 +1,21 @@
+{{/* Per-Series page: one address per Series, keyed "<site>:<series_id>" so the
+     list row is one hop from it. Everything here is a Series-level fact plus
+     the anonymous Reader count; the intervention forms a later ticket adds
+     land inside the empty .detail-grid, so this page edits nothing. */}}
+{{define "series-detail"}}
+<a class="ghost detail-back" href="/admin/series">← Series</a>
+<h1 class="detail-title">{{.Title}}</h1>
+<p class="detail-key">{{.Key}} · {{.Site}} · {{.Kind}}</p>
+{{if .Cover}}<div class="cover"><img src="{{.Cover}}" alt="" loading="lazy"></div>
+{{else}}<div class="cover"></div>{{end}}
+<div class="detail-meta">
+  <span>ch {{.Chapter}}</span>
+  <span>checked {{.Checked}}</span>
+  <span>{{.Readers}} readers</span>
+  {{if .Unpollable}}<span class="mark">unpollable</span>{{end}}
+  {{if .NoCover}}<span class="mark">no cover</span>{{end}}
+  {{if .Orphan}}<span class="mark">orphan</span>{{end}}
+  {{if .SightingRaised}}<span class="mark">sighting-raised</span>{{end}}
+</div>
+<div class="detail-grid"></div>
+{{end}}