Expand README into production template documentation

[Subhransu-De]

What to build

Turn the README from a quick-start file into useful documentation for a reusable production FastAPI template.

Acceptance criteria

• README documents local setup, Docker setup, testing, linting, type checking, and migrations.
• README documents environment variables and expected secrets.
• README documents OIDC provider setup at a template level.
• README documents deployment assumptions and production startup flow.
• README documents observability, health checks, and troubleshooting basics.
• README badge references existing workflows.

Blocked by

None - can start immediately.