feat(#113): 100% blob storage model — PostgreSQL metadata + DedupService blobs

BREAKING CHANGE: Storage model completely rewritten. All file/folder
metadata now lives in PostgreSQL (storage schema). File content stored
as content-addressable blobs via DedupService. Filesystem directories
are no longer used for user storage.

New components:
- storage.folders / storage.files / storage.trash_items (PG schema)
- FolderDbRepository: virtual folders backed by PG
- FileBlobReadRepository: file reads via PG metadata + dedup blobs
- FileBlobWriteRepository: file writes via PG metadata + dedup blobs
- TrashDbRepository: soft-delete trash using is_trashed flags

Removed legacy FS components (~5500 lines deleted):
- FolderFsRepository, FileFsReadRepository, FileFsWriteRepository
- CompositeFileRepository, ParallelFileProcessor
- IdMappingService, IdMappingOptimizer, FileMetadataCache
- BufferPool, FileSystemUtils, RepositoryErrors
- TrashFsRepository, FolderFsRepositoryTrash

DI rewired: build_app_state() now requires PgPool (no FS fallback).
FileUploadService.new_with_read() and FileRetrievalService.new_with_cache()
constructors added for blob model (no write-behind needed).

Closes #113

src/infrastructure/repositories/pg/mod.rs

@@ -11,6 +11,12 @@ mod settings_pg_repository;
 mod transaction_utils;
 mod user_pg_repository;
 
+// ── Blob-storage repositories ──
+pub mod file_blob_read_repository;
+pub mod file_blob_write_repository;
+pub mod folder_db_repository;
+pub mod trash_db_repository;
+
 pub use address_book_pg_repository::AddressBookPgRepository;
 pub use calendar_event_pg_repository::CalendarEventPgRepository;
 pub use calendar_pg_repository::CalendarPgRepository;
@@ -18,7 +24,11 @@ pub use contact_group_pg_repository::ContactGroupPgRepository;
 pub use contact_persistence_dto::*;
 pub use contact_pg_repository::ContactPgRepository;
 pub use favorites_pg_repository::FavoritesPgRepository;
+pub use file_blob_read_repository::FileBlobReadRepository;
+pub use file_blob_write_repository::FileBlobWriteRepository;
+pub use folder_db_repository::FolderDbRepository;
 pub use recent_items_pg_repository::RecentItemsPgRepository;
 pub use session_pg_repository::SessionPgRepository;
 pub use settings_pg_repository::SettingsPgRepository;
+pub use trash_db_repository::TrashDbRepository;
 pub use user_pg_repository::UserPgRepository;