Can't upgrade sink schema if using yaml source and metric configs

[0xgouda]

Describe the bug
One can't only specify --sink= option in the config upgrade command to upgrade the sink db only (the case where you are using yaml files for metrics and sources)

To Reproduce
Steps to reproduce the behavior:

1. Run ./pgwatch --sink=[your-sink-db-conn-str] config upgrade
2. Observe the help message stating missing --sources and --metrics flags

Expected behavior
The pgwatch config upgrade command should tolerate passing the --sink alone.

[pashagolub]

we want also to change pgwatch --version output to include expected schema version for sink:

Version info:
  Version:       5.0.0
  Config Schema: 00824
  Sink Schema:   01110
  Git Commit:    4312f1
  Built:         26.01.2025

[pashagolub]

also instead of ERROR we want WARN for unsupported upgrades, e..g

 ./pgwatch --sources=sources.yaml --metrics="../internal/metrics/metrics.yaml" --sink="postgresql://postgres@localhost/pgwatch_metrics" config upgrade

2026-01-07 16:11:36.818 [INFO] [sink:postgres] [db:postgres] initialising measurements database...
2026-01-07 16:11:36.819 [WARN] [sources.yaml] configuration storage does not support upgrade, skipping
2026-01-07 16:11:36.820 [WARN] [metrics.yaml] configuration storage does not support upgrade, skipping
...

[Mohamed-Elfardy]

Hi, I’m interested in working on this issue.
I’ve set up pgwatch locally, reproduced the behavior, and will start investigating the config upgrade flow and flag validation.
Please let me know if that sounds good.