feat(upload): add programmatic Ingest service and file-upload skill

Introduce upload/ingest as the single domain entry for storing files,
writing w_uploads records, and maintaining incremental stats. Refactor
HTTP UploadFile and delete handlers to delegate to ingest, fix stats
decrement ordering on Remove, and document usage in the file-upload skill.

internal/apps/upload/ingest/errors.go

@@ -0,0 +1,16 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package ingest
+
+import (
+	"errors"
+
+	"github.com/Rain-kl/Wavelet/internal/apps/upload/shared"
+)
+
+// ErrForbidden indicates the caller is not allowed to mutate the upload record.
+var ErrForbidden = errors.New("upload forbidden")
+
+// ErrStorageReadOnly indicates the storage backend is in migration read-only mode.
+var ErrStorageReadOnly = errors.New(shared.ErrStorageReadOnly)