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/main.go

@@ -16,6 +16,7 @@ import (
 	"booklib/internal/scanner"
 	"booklib/internal/seed"
 	"booklib/internal/store"
+	"booklib/internal/upload"
 )
 
 func main() {
@@ -39,11 +40,12 @@ func main() {
 		log.Fatalf("seed: %v", err)
 	}
 	rdb := redispkg.New(cfg.RedisURL)
-	sc := scanner.New(st, 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),
+	srv := &http.Server{Addr: cfg.Addr, Handler: api.NewRouter(cfg, st, rdb, sc, up),
 		ReadHeaderTimeout: 10 * time.Second}
 	go func() {
 		log.Printf("listening on %s", cfg.Addr)