feat: F5.5 large binary files as blobs on server F

From 16 KB a picture's or binary file's entry names its bytes by their
SHA-256 instead of holding them; the bytes are on F, content-addressed
(GET/POST /blobs with the session's grant, from the editor's origin, at
most 16 MiB). A document's blob side says where they are: the editor's
fetches and uploads, the room thread's asks F's main thread, and a start
waits for the ones asked for; without a side everything stays in the
document. Text stays Y.Text. A blob whose bytes are not there is never
written back empty. A blob F has not got yet is asked for again a few
times, and a running room reads late bytes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/server/test/harness.ts

@@ -16,7 +16,7 @@ import { expect } from 'vitest';
 import WebSocket from 'ws';
 import * as Y from 'yjs';
 import { emptyProject, writeProject } from '@mdg-filius/doc';
-import { startServer, type DocStore, type FiliusServer, type Log, type ServerOptions, type SessionStore } from '../src/index.js';
+import { PostgresBlobs, startServer, type DocStore, type PostgresStore, type FiliusServer, type Log, type ServerOptions, type SessionStore } from '../src/index.js';
 import { signTicket } from './tickets.js';
 
 export const TICKET_SECRET = 'test-ticket-secret';
@@ -38,6 +38,13 @@ export function memoryLog(): MemoryLog {
 
 export type TestServer = FiliusServer & { readonly log: MemoryLog; readonly base: string };
 
+/** F's blob store in the test database (the table made, once a file). */
+async function testBlobs(store: DocStore): Promise<PostgresBlobs> {
+  const blobs = new PostgresBlobs((store as PostgresStore).pool);
+  await blobs.migrate();
+  return blobs;
+}
+
 let roomThread: Promise<URL> | undefined;
 
 /** src/simWorker.ts bundled into a directory of its own, once a test file. */
@@ -63,6 +70,7 @@ export async function startTestServer(store: DocStore, sessions: SessionStore, o
     origins: [EDITOR_ORIGIN],
     secureCookie: false,
     workerUrl: await bundledRoomThread(),
+    blobs: await testBlobs(store),
     simThreads: 1,
     ...options,
   });