feat: pluggable storage backends (S3, Azure, local) with admin UI

Implement 4-phase external storage backends architecture:

Phase 1 - Foundation:
- BlobStorageBackend trait (application/ports/blob_storage_ports.rs)
- LocalBlobBackend: extracted all tokio::fs ops from DedupService
- S3BlobBackend: AWS SDK with custom endpoint support (MinIO, R2, B2)
- DedupService refactored to use Arc<dyn BlobStorageBackend>

Phase 2 - Admin Panel:
- StorageSettingsService with DB persistence + env override
- Storage tab in admin panel (backend selector, S3 form, provider presets)
- GET/PUT/POST endpoints for storage settings + connection test
- i18n keys (en/es) and BEM CSS

Phase 3 - Migration:
- MigrationBlobBackend decorator (dual-read: target-first + source fallback)
- Background migration job with parallel transfers + progress tracking
- Migration UI (progress bar, ETA, pause/resume/verify/complete)
- 6 admin API endpoints for migration lifecycle

Phase 4 - Enterprise Extras:
- CachedBlobBackend: LRU disk cache for remote backends
- EncryptedBlobBackend: AES-256-GCM at-rest encryption
- AzureBlobBackend: Azure Blob Storage support
- RetryBlobBackend: exponential backoff for transient errors
- Decorator composition in DI: retry → encryption → cache

All 223 tests passing, clippy clean, fmt verified.

src/application/services/mod.rs

@@ -18,6 +18,7 @@ pub mod nextcloud_login_flow_service;
 pub mod recent_service;
 pub mod search_service;
 pub mod share_service;
+pub mod storage_settings_service;
 pub mod storage_usage_service;
 pub mod trash_service;
 pub mod wopi_lock_service;