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/users.go

@@ -3,21 +3,18 @@ package handlers
 import (
 	"errors"
 	"net/http"
-	"strconv"
 	"time"
 
 	"github.com/gin-gonic/gin"
 	"github.com/jackc/pgx/v5"
 
 	"booklib/internal/auth"
+	"booklib/internal/ports"
 	"booklib/internal/store"
 )
 
-// isUnique is a convenience alias for store.IsUniqueViolation.
-func isUnique(e error) bool { return store.IsUniqueViolation(e) }
-
 func (h *H) ListUsers(c *gin.Context) {
-	users, e := h.st.ListUsers(c)
+	users, e := h.users.ListUsers(c)
 	if e != nil {
 		dbErr(c, e)
 		return
@@ -49,9 +46,9 @@ func (h *H) CreateUser(c *gin.Context) {
 		err(c, http.StatusInternalServerError, "internal", "hash")
 		return
 	}
-	id, e := h.st.CreateUser(c, req.Username, hp, req.Role)
+	id, e := h.users.CreateUser(c, req.Username, hp, req.Role)
 	if e != nil {
-		if isUnique(e) {
+		if ports.IsUniqueViolation(e) {
 			err(c, http.StatusConflict, "exists", "username taken")
 			return
 		}
@@ -62,9 +59,8 @@ func (h *H) CreateUser(c *gin.Context) {
 }
 
 func (h *H) DeleteUser(c *gin.Context) {
-	id, e := strconv.ParseInt(c.Param("id"), 10, 64)
-	if e != nil {
-		err(c, http.StatusBadRequest, "bad_request", "bad id")
+	id, ok := idParam(c)
+	if !ok {
 		return
 	}
 	if id == uid(c) {
@@ -72,7 +68,7 @@ func (h *H) DeleteUser(c *gin.Context) {
 		return
 	}
 	// B5: transactional last-admin check eliminates TOCTOU race.
-	if e := h.st.DeleteUser(c, id); e != nil {
+	if e := h.users.DeleteUser(c, id); e != nil {
 		if errors.Is(e, pgx.ErrNoRows) {
 			err(c, http.StatusNotFound, "not_found", "no such user")
 			return