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/uploads.go

@@ -19,7 +19,11 @@ import (
 const maxChunkBytes = 32 << 20
 
 func (h *H) UploadInit(c *gin.Context) {
-	lib, ok := h.getLibrary(c)
+	id, ok := idParam(c)
+	if !ok {
+		return
+	}
+	lib, ok := h.getLib(c, id)
 	if !ok {
 		return
 	}
@@ -75,7 +79,7 @@ func (h *H) UploadComplete(c *gin.Context) {
 		h.mapUploadErr(c, e)
 		return
 	}
-	lib, e := h.st.GetLibrary(c, libID)
+	lib, e := h.libs.GetLibrary(c, libID)
 	if e != nil {
 		dbErr(c, e)
 		return