Add x402 environment configuration for dev deployments

[crtahlin]

Summary

Enables x402 configuration for dev branch deployments via GitHub environment variables.

• Use GitHub staging environment for dev branch deployments
• Write x402 config from GitHub Variables to /opt/swarm_connect_dev.env
• Configure dev container to load env file for x402 settings

Closes #53

Variables to configure in staging environment

After merging, set these variables in the staging environment:

gh variable set X402_ENABLED --body "true" --env staging --repo datafund/swarm_connect
gh variable set X402_PAY_TO_ADDRESS --body "<wallet>" --env staging --repo datafund/swarm_connect
gh variable set X402_NETWORK --body "base-sepolia" --env staging --repo datafund/swarm_connect
gh variable set X402_FACILITATOR_URL --body "https://x402.org/facilitator" --env staging --repo datafund/swarm_connect
gh variable set X402_FREE_TIER_ENABLED --body "true" --env staging --repo datafund/swarm_connect

Test plan

• Merge to dev branch
• Set staging environment variables
• Verify deployment creates env file
• Verify dev gateway starts with x402 enabled
• Test protected endpoint returns 402 with payment requirements