fix: aggregate ledger costs before rounding

[EvanProgramming]

Summary

• aggregate frozen per-million pricing numerators from the SQLite ledger before converting to the stored sub-cent unit
• preserve immutable per-attempt pricing snapshots while avoiding request-boundary precision loss
• add short-call and mixed-usage regression coverage

Validation

• python -m unittest discover -s tests -p 'test_usage_ledger.py' -v\n- make check\n- make lint\n- make docs-check\n- make test (155 passed, including browser flow)\n- git diff --check\n\nFixes [P2] Per-request cent truncation erases cumulative cost for short calls #78