Cost API Full Filters + CSV Export

[kaissb]

Branch: feature/0.1.1-cost-api-filters-export
Version: v0.1.1 — Gate Completion

Build

1. GET /costs — filters: agent, model, from, to, group_by (agent|model|hour|day). Response: total_usd, total_requests, total_input_tokens, total_output_tokens, breakdown[].
2. GET /costs/export — same filters, returns text/csv with Content-Disposition header. Columns: agent, model, provider, requests, input_tokens, output_tokens, cost_usd.
3. Parameterized SQL queries in internal/storage/.

Tests

• Unit: filter by agent; filter by date; group_by hour/model; empty → zero totals; CSV valid headers.
• Integration (scripts/test-cost-api.sh): send requests from different agents; query with filters; download CSV.

Commit message

feat: add cost API filters and CSV export

- Full filter support (agent, model, date range, group_by)
- CSV export endpoint with proper Content-Disposition
- Parameterized SQL queries for all filters