backend: remove orphan series from the admin (ticket #155)

(*Store).RemoveSeries deletes one series row by (site, series_id) via a
plain parameterized DELETE; a bookmarks_series_fk violation outside
23503 is translated into the ErrSeriesHasBookmarks sentinel so no
driver type escapes the store. The caller reads the row's cover
address before the delete and reclaims it after: ReclaimCover's guard
cannot pass while a series row still points at the address.

POST /admin/series/{key}/remove answers the list row with the removed
row's fragment plus the heading re-rendered out of band at the fresh
count (HX-Reswap: delete removes the row through the same button that
swaps the refusal back in), and navigates from the detail page to the
No-Readers list (HX-Redirect for htmx, a 303 for plain clients). A
removal that races a fresh Bookmark is a refusal, not a 500: the row
re-renders at its new count with the fact spelled out. The control
renders only at zero Reader count on both surfaces, gated by hx-confirm
with the brief's copy.

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

@@ -34,6 +34,11 @@
   <div class="field"><a class="ghost act" hx-post="/admin/series/{{.Key}}/poll" hx-target="#detail-meta" hx-swap="outerHTML" href="#">Check now</a></div>
 </div>
 {{end}}
+{{if .CanRemove}}
+<div class="dform">
+  <div class="field"><button class="ghost danger" hx-post="/admin/series/{{.Key}}/remove" hx-target="#detail-meta" hx-confirm="Removes this series and its stored cover. No Reader has it bookmarked; one re-bookmarking it recreates the row.">Remove</button></div>
+</div>
+{{end}}
 {{end}}
 
 {{/* series-detail-meta is the meta line, and the answer a Check now or