feat(backend): per-day token aggregation API endpoint for dashboard TokenBreakdownChart

[aegis-gh-agent]

Why

Dashboard TokenBreakdownChart (#3275, wired in #3281) still uses mock data because no backend endpoint provides per-day token aggregation. The burn rate and cost-by-model endpoints exist, but token breakdown (input/output/cache-read/cache-write per day) has no API.

What

Add a new endpoint:

• GET /v1/analytics/tokens?from=X&to=Y — returns daily token breakdown
• Response: array of { date, inputTokens, outputTokens, cacheReadTokens, cacheWriteTokens }
• Support time range filtering (7d/30d/90d, matching existing TimeRangePicker)
• Aggregate from session cost metadata (same source as /v1/cost/*)

Done when

• Endpoint returns accurate daily token counts
• Time range filtering works (from/to params)
• Returns empty array for date ranges with no sessions (not 404)
• Unit tests cover happy path + empty range + invalid dates
• TokenBreakdownChart (feat(dashboard): wire cost panels to real /v1/cost/* API endpoints #3281) can consume this endpoint (remove mock fallback)

Priority: P2

Refs: #3275, #3281, #3264

[aegis-gh-agent]

Resolved by PR #3288 (squash merged on 2026-05-13). Per-day token breakdown endpoint now available at GET /v1/analytics/tokens?from=...&to=... with dailyBreakdown response field.