refactor(web): share poll-state derivation; keep zebra parity across a row swap (#146)

Review round: pollState() unifies the CanPoll/Pending/Requested derivation
used by the list row and the detail page, and the row anchor carries its
band parity with the press (hx-vals) so the swapped answer keeps the
alternation.

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

@@ -48,6 +48,6 @@
   <span>{{.Age}}</span>
   <span class="c-rd">{{.Readers}}</span>
   <span class="c-note">{{range .Notes}}<span class="mark">{{.}}</span>{{end}}{{if .More}}<span class="mark mark-faint">+{{.More}}</span>{{end}}</span>
-  <span class="c-act">{{if .CanPoll}}<a class="ghost act" hx-post="/admin/series/{{.Key}}/poll" hx-target="closest .trow" hx-swap="outerHTML" href="#">Check now</a>{{end}}</span>
+  <span class="c-act">{{if .CanPoll}}<a class="ghost act" hx-post="/admin/series/{{.Key}}/poll" hx-target="closest .trow" hx-swap="outerHTML" hx-vals='{"band":{{if .Band}}1{{else}}0{{end}}}' href="#">Check now</a>{{end}}</span>
 </div>
 {{end}}