feat: add organization AI spend overview endpoint - #31709

Description

Adds the experimental GET /api/experimental/organizations/{organization}/ai/spend endpoint that backs the new Overview tab of the organization AI spend page. It returns the organization's spend summary, daily spend, and a page of users with their per-group spend, using the queries from #31711.

Changes

• Add the organizationAISpend handler and ExperimentalClient.OrganizationAISpend:
  • Reuses the existing period handling (period_start/period_end together, at most 31 days, default to the current UTC month narrowed to retention). Bounds must be at UTC midnight; a default period narrowed by retention starts at the next UTC day.
  • Returns count, totals (spend_micros, median_spend_micros), daily (one bucket per UTC day, zeros included), and users (paginated with limit/offset, most spend first, with per-group spend).
  • Echoes the applied period, but every value covers it only through today, read in one read-only repeatable-read transaction so the values agree.
• Extract parseOrganizationAISpendPage, shared with /ai/spend/users.
• Add handler tests.

Closes AIGOV-728