feat: Add response models (PUT/POST/DELETE endpoints untouched)

[suchithh]

What

This PR adds proper response models for all endpoints to ensure the FastAPI documentation accurately represents the actual response structure. Follow-up to #231.

Changes

• Added HelloResponse model for root endpoint
• Added TokenResponse model for auth endpoints
• Added PingResponse model for /ping endpoint
• Added ValueCount model for /values endpoint
• Minor typo in an error response

These changes improve the documentation accuracy in the OpenAPI spec and potentially type safety for the API.

Fixes bug(s)

Fixes #118

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.69%. Comparing base (b9cda65) to head (298cd5d).
Report is 67 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
- Coverage   95.06%   94.69%   -0.37%     
==========================================
  Files           5        5              
  Lines         324      377      +53     
==========================================
+ Hits          308      357      +49     
- Misses         16       20       +4     

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[alexgarel]

Good idea, good you please add doc strings to all those models classes in another PR @suchithh

[suchithh]

@alexgarel On it!