feat: backfill thumbnails for existing image attachments on first startup

One-time async migration runs after server is listening — generates 800px
WebP thumbnails for all existing image attachments that lack one. Gated by
a persistent flag in instance_settings so it never re-runs.

packages/server/src/index.ts

@@ -5,8 +5,9 @@ import websocket from '@fastify/websocket';
 import multipart from '@fastify/multipart';
 import fastifyStatic from '@fastify/static';
 import { config } from './config.js';
-import { getDb } from './db/index.js';
+import { getDb, getRawDb } from './db/index.js';
 import { seedDatabase } from './db/seed.js';
+import { backfillThumbnails } from './db/migrate.js';
 import { authRoutes } from './routes/auth.js';
 import { userRoutes } from './routes/users.js';
 import { spaceRoutes } from './routes/spaces.js';
@@ -111,6 +112,11 @@ async function main(): Promise<void> {
     process.exit(1);
   }
 
+  // Fire-and-forget: backfill thumbnails for existing images (runs once)
+  backfillThumbnails(getRawDb(), config.uploadDir).catch(err => {
+    console.error('Thumbnail backfill failed (non-fatal):', err);
+  });
+
   const shutdown = async () => {
     console.log('Shutting down...');
     await app.close();