feat: add provider cost attribution

[RWDai]

Summary

• add provider_id/provider_name filters to usage aggregation repositories
• add GET /api/admin/usage/attribution with provider-scoped user attribution, selected metric sorting, share calculation, and Others bucket
• add Cost Analysis provider attribution UI and frontend API contract coverage
• fix review-blocking edge cases for legacy provider-name attribution and unbounded internal attribution totals before visible-limit sorting

Closes #637

Verification

• cargo fmt --all -- --check
• BINDGEN_EXTRA_CLANG_ARGS=-isystem/usr/lib/gcc/x86_64-linux-gnu/13/include cargo test -p aether-gateway usage_attribution
• npm run test:run -- src/api/tests/usage-contract.spec.ts
• npm run type-check

Note: local Rust tests require BINDGEN_EXTRA_CLANG_ARGS because this environment lacks clang and bindgen otherwise cannot find stddef.h. npm ci reported existing dependency advisories (1 moderate, 2 critical) unrelated to this change.