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/main.go

@@ -12,6 +12,7 @@ import (
 	"booklib/cmd/webui/api"
 	"booklib/internal/config"
 	"booklib/internal/db"
+	"booklib/internal/media"
 	"booklib/internal/redispkg"
 	"booklib/internal/scanner"
 	"booklib/internal/seed"
@@ -40,12 +41,13 @@ func main() {
 		log.Fatalf("seed: %v", err)
 	}
 	rdb := redispkg.New(cfg.RedisURL)
+	med := media.New(cfg, rdb)
 	up := upload.New(cfg.BooksDir, cfg.UploadMaxMB)
 	sc := scanner.New(st, cfg, rdb, up) // B16: sweep rides the scan ticker
 	go sc.Run(ctx)
 
 	serveErr := make(chan error, 1)
-	srv := &http.Server{Addr: cfg.Addr, Handler: api.NewRouter(cfg, st, rdb, sc, up),
+	srv := &http.Server{Addr: cfg.Addr, Handler: api.NewRouter(cfg, st, rdb, sc, med, up),
 		ReadHeaderTimeout: 10 * time.Second}
 	go func() {
 		log.Printf("listening on %s", cfg.Addr)