refactor(handlers): consume port interfaces, wire media+upload assembly (Task 25+26)

- H now holds only small port interfaces (UserStore/LibraryStore/BookStore/
  ProgressStore/BookmarkStore/RateLimiter/Scanner/Media/UploadSessions);
  NewRouter is the composition root distributing *store.Store and *redispkg.R
- ports.Media gains EnsurePage; ChaptersOf returns ports.Chapter (media's
  local duplicate dropped); *media.M now provably satisfies ports.Media;
  ports.UploadSessions gains LibraryID for root validation before Complete
- content.go: duplicated page-index cache + cover self-heal + page extract
  logic removed in favor of media service — same redis keys, same contract;
  path traversal check stays in handler (403 semantics preserved)
- getLibrary/getLibRow merged into getLib(c, id); idParam helper dedupes
  :id parsing; isUnique replaced by ports.IsUniqueViolation (Task 28 partial)
- main.go assembles media + upload and passes upload.U as scanner Sweeper

Full gate green: gofmt, vet, go test -p 1 (real PG+Redis, 0 skip)

backend/cmd/webui/handlers/auth_test.go

@@ -18,6 +18,7 @@ import (
 	"booklib/internal/auth"
 	"booklib/internal/config"
 	"booklib/internal/db"
+	"booklib/internal/media"
 	"booklib/internal/redispkg"
 	"booklib/internal/scanner"
 	"booklib/internal/store"
@@ -58,9 +59,10 @@ func setupAPI(t *testing.T) (*store.Store, *scanner.Scanner, http.Handler, strin
 	cfg.BooksDir = booksDir
 	cfg.CacheDir = t.TempDir()
 	rdb := redispkg.New(os.Getenv("REDIS_URL"))
+	med := media.New(cfg, rdb)
 	up := upload.New(cfg.BooksDir, cfg.UploadMaxMB)
 	sc := scanner.New(st, cfg, rdb, up)
-	r := api.NewRouter(cfg, st, rdb, sc, up)
+	r := api.NewRouter(cfg, st, rdb, sc, med, up)
 	if u := os.Getenv("REDIS_URL"); u != "" { // 测试卫生: 共享 redis 上重置登录限流桶, 防跨测试累计 429
 		if opt, e := redis.ParseURL(u); e == nil {
 			rc := redis.NewClient(opt)