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/dedup_ports.rs

@@ -28,16 +28,11 @@ pub struct BlobMetadataDto {
 #[derive(Debug, Clone)]
 pub enum DedupResultDto {
     /// New content was stored (first occurrence).
-    NewBlob {
-        hash: String,
-        size: u64,
-        blob_path: PathBuf,
-    },
+    NewBlob { hash: String, size: u64 },
     /// Content already existed; a reference was added instead.
     ExistingBlob {
         hash: String,
         size: u64,
-        blob_path: PathBuf,
         saved_bytes: u64,
     },
 }
@@ -57,13 +52,6 @@ impl DedupResultDto {
         }
     }
 
-    pub fn blob_path(&self) -> &Path {
-        match self {
-            DedupResultDto::NewBlob { blob_path, .. } => blob_path,
-            DedupResultDto::ExistingBlob { blob_path, .. } => blob_path,
-        }
-    }
-
     pub fn was_deduplicated(&self) -> bool {
         matches!(self, DedupResultDto::ExistingBlob { .. })
     }