fix: use env_file for L2 server config

- Add env_file support for L2_SERVER and L2_DOMAIN
- Fix typo in L2_SERVER URL (ros-ash -> rose-ash)
- Add .env.example with required variables

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,7 @@
+# L1 Server Configuration
+
+# L2 server URL (for authentication and publishing)
+L2_SERVER=https://artdag.rose-ash.com
+
+# L2 domain for ActivityPub actor IDs (e.g., @user@domain)
+L2_DOMAIN=artdag.rose-ash.com