feat(admin): implement provider-specific storage configuration pages

Refactor the admin storage backend creation and editing flows to use
provider-specific pages (e.g., `/admin/storage/new/sftp`) instead of a
single generic form. This ensures only relevant fields are rendered for
each storage provider (such as SFTP, S3, or WebDAV).

Additionally:
- Prevent mutation of the storage provider type during backend edits.
- Add comprehensive unit tests for provider-specific rendering, edit
  validation, and CSRF/admin route protection.

backend/libs/services/upload_test.go

@@ -172,6 +172,157 @@ func TestSFTPStorageConfigValidation(t *testing.T) {
 	}
 }
 
+func TestStorageUpdateRejectsProviderMutation(t *testing.T) {
+	service := newTestUploadService(t)
+	cfg, err := service.Storage().CreateBackend(StorageBackendConfig{
+		Provider: StorageProviderSFTP,
+		Name:     "SFTP",
+		Host:     "files.example.test",
+		Username: "warpbox",
+		Password: "secret",
+	})
+	if err != nil {
+		t.Fatalf("CreateBackend returned error: %v", err)
+	}
+	if _, err := service.Storage().UpdateBackend(cfg.ID, StorageBackendConfig{
+		Provider:  StorageProviderS3,
+		Name:      "Mutated",
+		Endpoint:  "https://s3.example.test",
+		Bucket:    "bucket",
+		AccessKey: "access",
+		SecretKey: "secret",
+		UseSSL:    true,
+	}); err == nil {
+		t.Fatalf("UpdateBackend allowed provider mutation")
+	}
+	stored, err := service.Storage().BackendConfig(cfg.ID)
+	if err != nil {
+		t.Fatalf("BackendConfig returned error: %v", err)
+	}
+	if stored.Provider != StorageProviderSFTP || stored.Type != StorageBackendSFTP {
+		t.Fatalf("provider/type mutated despite error: %+v", stored)
+	}
+	if _, err := service.Storage().UpdateBackend(cfg.ID, StorageBackendConfig{
+		Provider: StorageProviderSFTP,
+		Type:     StorageBackendS3,
+		Name:     "Mutated",
+		Host:     "files.example.test",
+		Username: "warpbox",
+		Password: "secret",
+	}); err == nil {
+		t.Fatalf("UpdateBackend allowed type mutation")
+	}
+}
+
+func TestStorageUpdatePreservesSecretsWhenBlank(t *testing.T) {
+	service := newTestUploadService(t)
+	cfg, err := service.Storage().CreateBackend(StorageBackendConfig{
+		Provider:   StorageProviderSFTP,
+		Name:       "SFTP",
+		Host:       "files.example.test",
+		Username:   "warpbox",
+		Password:   "secret",
+		PrivateKey: "private-key",
+		HostKey:    "host-key",
+	})
+	if err != nil {
+		t.Fatalf("CreateBackend returned error: %v", err)
+	}
+	updated, err := service.Storage().UpdateBackend(cfg.ID, StorageBackendConfig{
+		Provider: StorageProviderSFTP,
+		Name:     "SFTP renamed",
+		Host:     "files.example.test",
+		Username: "warpbox",
+	})
+	if err != nil {
+		t.Fatalf("UpdateBackend returned error: %v", err)
+	}
+	if updated.Password != "secret" || updated.PrivateKey != "private-key" || updated.HostKey != "host-key" {
+		t.Fatalf("blank secret fields were not preserved: %+v", updated)
+	}
+}
+
+func TestContaboUpdateKeepsTLSAndPathStyleLocked(t *testing.T) {
+	service := newTestUploadService(t)
+	cfg, err := service.Storage().CreateBackend(StorageBackendConfig{
+		Provider:  StorageProviderContabo,
+		Name:      "Contabo",
+		Endpoint:  "https://eu2.contabostorage.com",
+		Bucket:    "My Main Bucket",
+		AccessKey: "access",
+		SecretKey: "secret",
+	})
+	if err != nil {
+		t.Fatalf("CreateBackend returned error: %v", err)
+	}
+	updated, err := service.Storage().UpdateBackend(cfg.ID, StorageBackendConfig{
+		Provider:  StorageProviderContabo,
+		Name:      "Contabo",
+		Endpoint:  "https://eu2.contabostorage.com",
+		Bucket:    "My Main Bucket",
+		AccessKey: "access",
+		SecretKey: "secret",
+		UseSSL:    false,
+		PathStyle: false,
+	})
+	if err != nil {
+		t.Fatalf("UpdateBackend returned error: %v", err)
+	}
+	if !updated.UseSSL || !updated.PathStyle {
+		t.Fatalf("contabo TLS/path-style were not locked: %+v", updated)
+	}
+}
+
+func TestStorageSpeedTestRequiresConnectionAndRuns(t *testing.T) {
+	service := newTestUploadService(t)
+	if _, err := service.Storage().StartSpeedTest(StorageBackendLocal, StorageSpeedModeSmall); err == nil {
+		t.Fatalf("StartSpeedTest allowed speed test before connection test")
+	}
+	if _, err := service.Storage().TestBackend(StorageBackendLocal); err != nil {
+		t.Fatalf("TestBackend local returned error: %v", err)
+	}
+	test, err := service.Storage().StartSpeedTest(StorageBackendLocal, StorageSpeedModeSmall)
+	if err != nil {
+		t.Fatalf("StartSpeedTest returned error: %v", err)
+	}
+	service.Storage().RunSpeedTest(testContext(), test.ID)
+	tests, err := service.Storage().ListSpeedTests(StorageBackendLocal, 10)
+	if err != nil {
+		t.Fatalf("ListSpeedTests returned error: %v", err)
+	}
+	if len(tests) != 1 {
+		t.Fatalf("speed tests len = %d, want 1", len(tests))
+	}
+	got := tests[0]
+	if got.Status != StorageSpeedStatusDone || got.ProgressPercent != 100 || got.Stage != "complete" || got.BytesWritten == 0 || got.BytesRead == 0 || got.FilesWritten == 0 {
+		t.Fatalf("speed test did not complete with metrics: %+v", got)
+	}
+}
+
+func TestCustomStorageSpeedTestUsesRequestedFiles(t *testing.T) {
+	service := newTestUploadService(t)
+	if _, err := service.Storage().TestBackend(StorageBackendLocal); err != nil {
+		t.Fatalf("TestBackend local returned error: %v", err)
+	}
+	test, err := service.Storage().StartSpeedTestWithOptions(StorageBackendLocal, StorageSpeedTestOptions{
+		Mode:             StorageSpeedModeCustom,
+		CustomFileCount:  3,
+		CustomFileSizeMB: 0.001,
+	})
+	if err != nil {
+		t.Fatalf("StartSpeedTestWithOptions returned error: %v", err)
+	}
+	service.Storage().RunSpeedTest(testContext(), test.ID)
+	tests, err := service.Storage().ListSpeedTests(StorageBackendLocal, 10)
+	if err != nil {
+		t.Fatalf("ListSpeedTests returned error: %v", err)
+	}
+	got := tests[0]
+	if got.Mode != StorageSpeedModeCustom || got.CustomFileCount != 3 || got.CustomFileSizeMB != 0.001 || got.FilesWritten != 3 || got.Status != StorageSpeedStatusDone {
+		t.Fatalf("custom speed test did not use requested files: %+v", got)
+	}
+}
+
 func TestSMBAndWebDAVStorageConfigValidation(t *testing.T) {
 	service := newTestUploadService(t)
 	smb, err := service.Storage().CreateS3Backend(StorageBackendConfig{