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/internal/scanner/scanner.go

@@ -19,17 +19,23 @@ import (
 	"booklib/internal/store"
 )
 
+// Sweeper 是 scanner 每轮顺手调用的清理钩子;upload.U 满足它(B16)。
+type Sweeper interface {
+	Sweep(ctx context.Context) error
+}
+
 type Scanner struct {
-	st  *store.Store
-	cfg *config.Config
-	rdb *redispkg.R
+	st       *store.Store
+	cfg      *config.Config
+	rdb      *redispkg.R
+	sweepers []Sweeper
 	// B9-②: per-library single-flight — concurrent scan triggers for the same
 	// library are merged into one execution, even without redis.
 	flights sync.Map // map[int64]*sync.WaitGroup
 }
 
-func New(st *store.Store, cfg *config.Config, rdb *redispkg.R) *Scanner {
-	return &Scanner{st: st, cfg: cfg, rdb: rdb}
+func New(st *store.Store, cfg *config.Config, rdb *redispkg.R, sweepers ...Sweeper) *Scanner {
+	return &Scanner{st: st, cfg: cfg, rdb: rdb, sweepers: sweepers}
 }
 
 func (s *Scanner) Run(ctx context.Context) {
@@ -40,6 +46,11 @@ func (s *Scanner) Run(ctx context.Context) {
 		case <-ctx.Done():
 			return
 		case <-t.C:
+			for _, sw := range s.sweepers { // B16: 上传会话清扫随扫描周期跑
+				if err := sw.Sweep(ctx); err != nil {
+					log.Printf("scan: sweep: %v", err)
+				}
+			}
 			libs, err := s.st.ListLibraries(ctx)
 			if err != nil {
 				log.Printf("scan: list libraries: %v", err)