Improve config loading warnings and prefer mongo_uri as source of database name

[ml-evs]

• Only warn about a missing config file if the user actually tried to set a non-default config file (otherwise assume things are controlled by env vars)
• Override the setting of the MongoDB database name in mongo_database with the fully qualified value in mongo_uri, when present.

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.67%. Comparing base (f34cff3) to head (30d2973).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
optimade/server/config.py	81.25%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2183   +/-   ##
=======================================
  Coverage   90.67%   90.67%           
=======================================
  Files          75       75           
  Lines        4845     4858   +13     
=======================================
+ Hits         4393     4405   +12     
- Misses        452      453    +1     

Flag	Coverage Δ
project	90.67% <81.25%> (+<0.01%)	⬆️
validator	90.67% <81.25%> (+<0.01%)	⬆️

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