Split backend into internal packages by responsibility

All Go files lived flat in backend/ as one package main. Move store,
latest-chapter polling, sessions, HTTP middleware, the JSON API, the
userscript handler, and the web UI (with its templates/static assets)
into backend/internal/{store,latest,session,httpmw,api,userscript,web},
each with an exported API. main.go becomes the composition root wiring
them into newRouter; root-level tests cover the assembled router while
package-local tests cover unit behavior. Update Dockerfile/.dockerignore
for the new internal/ tree and CLAUDE.md to describe the layout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/web/templates/list.html

@@ -0,0 +1,22 @@
+{{define "list"}}
+{{if .Items}}
+  {{range .Items}}{{template "card" .}}{{end}}
+  {{/* The client filter only hides cards, so without this the list area goes
+       blank on a query that matches nothing. filter.js fills in the query and
+       unhides it; it lives inside #list so a tab swap re-creates it. */}}
+  <div class="empty" id="no-match" hidden>
+    <strong>No titles match “<span class="no-match-q"></span>”.</strong>
+    <button type="button" class="clear-search">Clear search</button>
+  </div>
+{{else if eq .Tab "fav"}}
+  <div class="empty"><strong>No favourites yet.</strong><p>Star a series to pin it here.</p></div>
+{{else if eq .Tab "new"}}
+  <div class="empty"><strong>Nothing new.</strong><p>Every series is caught up to its latest chapter.</p></div>
+{{else if eq .Tab "archived"}}
+  <div class="empty"><strong>Nothing archived.</strong><p>Shelve a series to park it here — it keeps getting checked for new chapters.</p></div>
+{{else if eq .Tab "finished"}}
+  <div class="empty"><strong>Nothing finished yet.</strong><p>Mark a series finished and it moves out of your reading list.</p></div>
+{{else}}
+  <div class="empty"><strong>Nothing here yet.</strong><p>Bookmarks appear once the userscript records a chapter.</p></div>
+{{end}}
+{{end}}