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/application/services/file_retrieval_service.rs

@@ -56,6 +56,21 @@ impl FileRetrievalService {
         }
     }
 
+    /// Constructor for blob-storage model: read + content cache + transcode.
+    /// No write-behind needed — dedup handled at the repository layer.
+    pub fn new_with_cache(
+        file_read: Arc<dyn FileReadPort>,
+        content_cache: Arc<dyn ContentCachePort>,
+        transcode: Arc<dyn ImageTranscodePort>,
+    ) -> Self {
+        Self {
+            file_read,
+            write_behind: None,
+            content_cache: Some(content_cache),
+            transcode: Some(transcode),
+        }
+    }
+
     // ── private helpers ──────────────────────────────────────────
 
     /// Try to transcode image content to WebP and return transcoded variant.