ref(cogs): Use usecase directly as app_feature

[jan-auer]

Usecases are created on-the-fly by clients, and by design we want to avoid code changes to support new ones, keeping config overhead and misconfiguration risk low. The hard-coded usecase-to-app_feature mapping contradicts this: every new usecase silently falls back to "shared", losing COGS attribution until someone updates the match arm. Technically, "shared" is incorrect here, as this is not actually shared cost.

Since there is no cross-service consistency of app_feature values anyway, the usecase name is already the right granularity. Use it directly.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.45%. Comparing base (4b0fe8f) to head (c165adf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #508      +/-   ##
==========================================
- Coverage   87.47%   87.45%   -0.02%     
==========================================
  Files          86       86              
  Lines       13912    13903       -9     
==========================================
- Hits        12169    12159      -10     
- Misses       1743     1744       +1     

Components	Coverage Δ
Rust Backend	92.25% <100.00%> (-0.01%)	⬇️
Rust Client	80.06% <ø> (ø)
Python Client	88.51% <ø> (ø)

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