feat(uploads): add native resumable upload support

Implement a native chunked resumable upload API and frontend integration
to support reliable large file uploads.

Changes include:
- Added a 3-step resumable upload API flow (create session, upload chunks, complete session).
- Introduced configuration options for chunk size, retention hours, and toggling the feature.
- Updated the frontend to utilize resumable uploads with progress tracking.
- Configured temporary chunk storage under `data/tmp/uploads` with automatic cleanup.
- Documented the API flow and configuration in the README.

backend/libs/config/config_test.go

@@ -68,4 +68,13 @@ func TestLoadDefaultsUseLargeUploadFriendlyTimeouts(t *testing.T) {
 	if cfg.WriteTimeout != 0 {
 		t.Fatalf("WriteTimeout = %s, want 0 for long uploads", cfg.WriteTimeout)
 	}
+	if !cfg.ResumableUploadsEnabled {
+		t.Fatalf("ResumableUploadsEnabled = false, want true")
+	}
+	if cfg.ResumableChunkSize != 8*1024*1024 {
+		t.Fatalf("ResumableChunkSize = %d, want 8 MiB", cfg.ResumableChunkSize)
+	}
+	if cfg.ResumableRetention != 24*time.Hour {
+		t.Fatalf("ResumableRetention = %s, want 24h", cfg.ResumableRetention)
+	}
 }