Fix: bind to 0.0.0.0 for Fly.io proxy

SvelteKit adapter-node needs HOST=0.0.0.0 to accept connections
from Fly's proxy.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

fly.toml

···
   CONTENT_DB_PATH = '/litefs/content.db'
   LOCAL_DB_PATH = '/data/local.db'
   NODE_ENV = 'production'
+  HOST = '0.0.0.0'
 
 # Single volume for both LiteFS data and local DB
 [[mounts]]