feat(upload): extract upload subsystem + move sweep to scanner ticker (B16)

- internal/upload owns chunked-upload domain logic (fingerprint resume,
  part tmp+rename, assemble, session sweep) and the shared UniquePath
  helper used by both single-file and chunked completion paths
- handlers/uploads.go is now HTTP-only: bind params, call upload.U, map
  sentinel errors to the unchanged status/code/message contract
- B16: session sweep moved out of the UploadInit request path onto the
  scanner ticker via a Sweeper hook (upload.U satisfies it)
- unit tests for the package without PG/Redis; contract pinned by the
  existing handler integration tests (all green)

backend/cmd/webui/handlers/libraries.go

@@ -136,12 +136,12 @@ func (h *H) Upload(c *gin.Context) {
 		return
 	}
 	// B12: retry on O_EXCL collision — concurrent uploads with the same name
-	// can both get the same candidate from uniquePath (stat-then-create race).
+	// can both get the same candidate from UniquePath (stat-then-create race).
 	var dst, tmp string
 	var out *os.File
 	for attempt := 0; attempt < 5; attempt++ {
 		var e error
-		dst, e = h.uniquePath(root, name)
+		dst, e = h.up.UniquePath(root, name)
 		if e != nil {
 			err(c, http.StatusForbidden, "forbidden", e.Error())
 			return
@@ -190,28 +190,3 @@ func (h *H) Upload(c *gin.Context) {
 	}
 	c.JSON(http.StatusAccepted, gin.H{"accepted": true, "path": strings.TrimPrefix(dst, root+string(os.PathSeparator))})
 }
-
-// uniquePath 清洗后的 name 必须仍在 root 内;重名加 " (n)" 后缀
-func (h *H) uniquePath(root, name string) (string, error) {
-	ext := filepath.Ext(name)
-	base := strings.TrimSuffix(name, ext)
-	for i := 0; ; i++ {
-		cand := base + ext
-		if i > 0 {
-			cand = base + " (" + strconv.Itoa(i) + ")" + ext
-		}
-		p := filepath.Join(root, cand)
-		if filepath.Clean(p) != filepath.Join(root, filepath.Clean(cand)) ||
-			!strings.HasPrefix(filepath.Clean(p), root+string(os.PathSeparator)) {
-			return "", os.ErrInvalid
-		}
-		if _, e := os.Stat(p); os.IsNotExist(e) {
-			return p, nil
-		} else if e != nil {
-			return "", e
-		}
-		if i > 999 {
-			return "", os.ErrExist
-		}
-	}
-}