Disable rate limiter memory fallback in production

[silentgeckoaudit3801]

Closes #194.

Summary

• configure the module-level SlowAPI limiter with the Redis storage URI instead of leaving storage implicit
• disable in_memory_fallback_enabled when ENV_VERSION=PROD, while preserving the fallback in development
• fail production startup by pinging Redis during FastAPI lifespan before serving requests
• add regression coverage for PROD/DEV fallback policy and startup Redis ping behavior

Validation

• Parsed the changed Python files with ast.parse using the bundled Codex Python runtime.
• GitHub compare reports this branch is 0 commits behind main.

Testing

• Not run as a full local project suite because this workspace is avoiding dependency installation and project toolchain execution.