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/api/router.go

@@ -10,11 +10,12 @@ import (
 	"booklib/internal/redispkg"
 	"booklib/internal/scanner"
 	"booklib/internal/store"
+	"booklib/internal/upload"
 )
 
-func NewRouter(cfg *config.Config, st *store.Store, rdb *redispkg.R, sc *scanner.Scanner) *gin.Engine {
+func NewRouter(cfg *config.Config, st *store.Store, rdb *redispkg.R, sc *scanner.Scanner, up *upload.U) *gin.Engine {
 	gin.SetMode(gin.ReleaseMode)
-	h := handlers.New(cfg, st, rdb, sc)
+	h := handlers.New(cfg, st, rdb, sc, up)
 	r := gin.New()
 	if e := r.SetTrustedProxies(cfg.TrustedProxies); e != nil {
 		panic(e)