fix: gracefully error when osa.yaml config file is missing

[rorybyrne]

Summary

When the server starts without an osa.yaml config file (or OSA_CONFIG_FILE pointing to a non-existent file), it silently runs with 0 sources configured instead of providing a clear error message.

Current Behavior

TriggerInitialSourceRun: checking 0 configured sources

No warning or error that the config file is missing.

Expected Behavior

• Clear error message if config file is required but missing
• Or a warning log if running without config is allowed but unusual

Context

Discovered during docker-compose dev setup where bind mount overwrites the baked-in config.