Remove hardcoded secrets from public repo

- Replace hardcoded POSTGRES_PASSWORD, ADMIN_TOKEN, and L1 host IP
  with env var references in docker-compose.yml
- Remove default password fallback from database.py and app/config.py
- Update .env.example with required POSTGRES_PASSWORD, ADMIN_TOKEN, L1_HOST
- Update README to mark DATABASE_URL as required

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -1,5 +1,15 @@
 # L1 Server Configuration
 
+# PostgreSQL password (REQUIRED - no default)
+POSTGRES_PASSWORD=changeme-generate-with-openssl-rand-hex-16
+
+# Admin token for purge operations (REQUIRED - no default)
+# Generate with: openssl rand -hex 32
+ADMIN_TOKEN=changeme-generate-with-openssl-rand-hex-32
+
+# L1 host IP/hostname for GPU worker cross-VPC access
+L1_HOST=your-l1-server-ip
+
 # This L1 server's public URL (sent to L2 when publishing)
 L1_PUBLIC_URL=https://l1.artdag.rose-ash.com