ci: deploy to FastAPI Cloud with a deploy token

[mtauha]

Replaces the FastAPI Cloud GitHub integration (disconnected after the repo moved to the psxdata org) with a token-based deploy from Actions.

• New Deploy workflow runs fastapi deploy after the CI workflow succeeds on a push to main, and can be run manually (workflow_dispatch).
• Deploys the exact commit CI tested; one deploy at a time (concurrency group, no cancellation).
• Needs repo secrets FASTAPI_CLOUD_TOKEN (deploy token from the app's Deploy Tokens page) and FASTAPI_CLOUD_APP_ID.