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/main_test.go

@@ -10,6 +10,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"mangabm/backend/internal/store"
 )
 
 func TestLoadLatestPollDefaults(t *testing.T) {
@@ -148,7 +150,7 @@ func TestPutStatusValidation(t *testing.T) {
 			if tc.want != http.StatusOK {
 				return
 			}
-			var got Bookmark
+			var got store.Bookmark
 			if err := json.Unmarshal(rr.Body.Bytes(), &got); err != nil {
 				t.Fatalf("decode: %v", err)
 			}
@@ -187,7 +189,7 @@ func TestPutOmittedStatusPreservesArchivedAndAppliesProgress(t *testing.T) {
 		t.Fatalf("status = %d, want 200 (body %s)", rr.Code, rr.Body.String())
 	}
 
-	var got Bookmark
+	var got store.Bookmark
 	if err := json.Unmarshal(rr.Body.Bytes(), &got); err != nil {
 		t.Fatalf("decode: %v", err)
 	}