Sync 5: attachment blob sync — client id + content hash (M8)

Let native clients sync attachment blobs deterministically:
- note_attachments gains sha256 (migration 0018, nullable, no backfill).
  The delta feed's attachment metadata now carries size + sha256 so a
  client knows exactly which blobs it already has (dedupe) and can verify
  integrity after download.
- Upload accepts an optional client-supplied attachment id (multipart form
  field), so a file attached offline keeps its identity across sync;
  re-uploading an id the note already has is an idempotent no-op. The
  server hashes the stored bytes (sha256) on upload. Download by id already
  exists (owner/shared scoped).

Frontend Attachment type carries the new optional size/sha256.

(Still image-only mimes — broadening to any-file is task 1900. Blob sync
behavior is operator-verified on deploy.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/stores/notes.ts

@@ -27,6 +27,8 @@ export interface Attachment {
   id: string;
   url: string;
   mime: string;
+  size?: number;
+  sha256?: string | null;
 }
 
 // A past version of a note's title+body (version history).