Get Repo Costs

Per-repo cost breakdown.

Aggregates two data sources:

• credit_transactions (this user, scoped by repo_id) - billing-side spend in cents, broken down by transaction type, with 7d / 30d / lifetime windows.
• llm_costs.json artifact (engine-side per-operation telemetry) when present. The OSS indexer doesn't currently emit this artifact for every snapshot, so the response includes available=False rather than 500-ing when it's missing.

Auth: anyone can fetch the totals for a public snapshot (transactions are filtered by user_id, so anonymous callers see zeros for the spend half but the engine telemetry is still useful as a public stat). Private snapshots follow the standard owner-only rule.