Improve startup logging

[obazoud]

Is your feature request related to a problem? Please describe.
The application startup should provide more detailed logs to help with configuration and troubleshooting. Currently, when a Redis connection fails, the logs only show EOF or dial tcp://xx.xx.xx.xx, which is not clear enough.

Describe the solution you'd like
Add explicit and structured logs during startup to make configuration and connection issues easier to diagnose.
It could be:

• Display the path to the configuration file (if present)
• Show the values of environment variables related to Outpost (excluding secrets)
• Log the result of configuration merging and resolution
• Add an explicite logs indicating the success or failure of checks (dlq queues, ...), external connections (e.g., Redis, PostgreSQL, open a subscription, ...)
  ...

Describe alternatives you've considered
N/A

Additional context
N/A

[leggetter]

There is some improved logging in #465

But I agree we need to have a focused issue on this.