feat(dedup): CDC sub-file deduplication with FastCDC + parallel chunk storage + dedup skip

- Replace whole-file SHA-256 dedup with FastCDC 2020 content-defined chunking
  (min 64KB, avg 256KB, max 1MB) + BLAKE3 hashing
- Add chunk_manifests table (file_hash → chunk_hashes[] + chunk_sizes[])
- Add put_blob_from_bytes to BlobStorageBackend trait (all 7 backends)
- 3-phase store_chunks pipeline:
  Phase 0: batch-check existing chunks (single PG query)
  Phase 1: selective disk read (skip existing chunks entirely)
  Phase 2: parallel upload with buffer_unordered(8)
- CDC-aware read_blob_stream and read_blob_range_stream with legacy fallback
- Transactional manifest + chunk ref-count cascade on remove_reference
- 12 CDC tests (determinism, reassembly, contiguity, sub-file dedup, etc.)
- Update deduplication.md to reflect new architecture

src/application/ports/blob_storage_ports.rs

@@ -54,6 +54,12 @@ pub trait BlobStorageBackend: Send + Sync + 'static {
     /// without overwriting.  Returns the number of bytes stored.
     fn put_blob(&self, hash: &str, source_path: &Path) -> BoxFut<'_, Result<u64, DomainError>>;
 
+    /// Store a blob from in-memory bytes (used by CDC chunk storage).
+    ///
+    /// Must be **idempotent**: if the blob already exists the call succeeds
+    /// without overwriting.  Returns the number of bytes stored.
+    fn put_blob_from_bytes(&self, hash: &str, data: Bytes) -> BoxFut<'_, Result<u64, DomainError>>;
+
     /// Stream the full blob content in chunks.
     fn get_blob_stream(&self, hash: &str) -> BoxFut<'_, Result<BlobStream, DomainError>>;