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/userscript/userscript_test.go

@@ -0,0 +1,115 @@
+package userscript
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+const testToken = "s3cret-token"
+
+// sampleScript is a stand-in for the real userscript: a metadata block with a
+// @version line, plus a body that must survive the rewrite untouched.
+const sampleScript = `// ==UserScript==
+// @name         Manga Bookmark Sync
+// @version      1.5.0
+// @match        https://asurascans.com/*
+// ==/UserScript==
+(function () { "use strict"; })();
+`
+
+// writeScript drops a userscript in a temp dir with a known mtime and returns
+// its path plus the version string the handler is expected to stamp.
+func writeScript(t *testing.T, body string) (path, wantVersion string) {
+	t.Helper()
+	path = filepath.Join(t.TempDir(), "manga-bookmark.user.js")
+	if err := os.WriteFile(path, []byte(body), 0o644); err != nil {
+		t.Fatalf("write script: %v", err)
+	}
+	mod := time.Date(2026, 7, 28, 16, 42, 0, 0, time.UTC)
+	if err := os.Chtimes(path, mod, mod); err != nil {
+		t.Fatalf("chtimes: %v", err)
+	}
+	return path, "2026.07.28.1642"
+}
+
+// newTestMux registers Handler the same way main.go's router does, without
+// pulling in the store or the rest of the app.
+func newTestMux(token, path string) http.Handler {
+	mux := http.NewServeMux()
+	mux.HandleFunc("GET /u/{token}/manga-bookmark.user.js", Handler(token, path))
+	return mux
+}
+
+func getScript(t *testing.T, srv http.Handler, token string) *httptest.ResponseRecorder {
+	t.Helper()
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, httptest.NewRequest(http.MethodGet, "/u/"+token+"/manga-bookmark.user.js", nil))
+	return rr
+}
+
+func TestUserscriptServedWithStampedVersion(t *testing.T) {
+	path, wantVersion := writeScript(t, sampleScript)
+	rr := getScript(t, newTestMux(testToken, path), testToken)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if ct := rr.Header().Get("Content-Type"); !strings.HasPrefix(ct, "text/javascript") {
+		t.Errorf("Content-Type = %q, want text/javascript", ct)
+	}
+	if cc := rr.Header().Get("Cache-Control"); cc != "no-cache" {
+		t.Errorf("Cache-Control = %q, want no-cache", cc)
+	}
+	body := rr.Body.String()
+	if !strings.Contains(body, "// @version      "+wantVersion) {
+		t.Errorf("body has no stamped version %q:\n%s", wantVersion, body)
+	}
+	if strings.Contains(body, "1.5.0") {
+		t.Errorf("body still carries the file's own version:\n%s", body)
+	}
+	// Everything outside the @version line is served verbatim.
+	if !strings.Contains(body, `(function () { "use strict"; })();`) {
+		t.Errorf("body was altered beyond the version line:\n%s", body)
+	}
+	if !strings.Contains(body, "// @name         Manga Bookmark Sync") {
+		t.Errorf("metadata block was altered:\n%s", body)
+	}
+}
+
+// The empty-token case ("/u//manga-bookmark.user.js") is covered at the
+// router level (see backend's guardEmptyUserscriptToken): ServeMux 307s it to
+// "/u/manga-bookmark.user.js" before this handler's own token check ever runs.
+func TestUserscriptWrongTokenIs404(t *testing.T) {
+	path, _ := writeScript(t, sampleScript)
+	srv := newTestMux(testToken, path)
+	for _, tok := range []string{"wrong", testToken + "x", testToken[:3]} {
+		if got := getScript(t, srv, tok).Code; got != http.StatusNotFound {
+			t.Errorf("token %q: status = %d, want 404", tok, got)
+		}
+	}
+}
+
+func TestUserscriptMissingFileIs404(t *testing.T) {
+	srv := newTestMux(testToken, filepath.Join(t.TempDir(), "absent.user.js"))
+	if got := getScript(t, srv, testToken).Code; got != http.StatusNotFound {
+		t.Fatalf("status = %d, want 404", got)
+	}
+}
+
+func TestUserscriptWithoutVersionLineServedUnmodified(t *testing.T) {
+	const noVersion = "// ==UserScript==\n// @name x\n// ==/UserScript==\nconsole.log(1);\n"
+	path, _ := writeScript(t, noVersion)
+	rr := getScript(t, newTestMux(testToken, path), testToken)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if rr.Body.String() != noVersion {
+		t.Fatalf("body = %q, want it unmodified", rr.Body.String())
+	}
+}