feat: per-user AI usage breakdown in the org summary

[akash4550]

Problem

The org-scoped AI usage endpoint (GET /api/v1/analytics/ai-usage) reports totals plus feature and provider breakdowns, but not who drives usage. With cost estimation live (PRs #27–#28), the operator still cannot answer "which user is burning tokens?" without raw SQL.

Solution

Add requestsByUser to the existing org-scoped summary:

"requestsByUser": [
  { "userId": "u-1", "requests": 3, "successes": 3, "failures": 0,
    "totalTokens": 45, "totalCostUsd": 0.9 },
  ...
]

• Per-user requests / successes / failures / totalTokens / totalCostUsd, org-scoped like every other breakdown (two additional groupBy queries on userId, all + success — same pattern as feature/provider).
• Sorted by spend descending (highest cost first) — mirrors the platform org breakdown's sort intent; the first question is "who costs the most?".
• Read-only, additive, no schema change, no UI change.

Tests

ai-usage.service.test.ts extended (2 new tests + updated chained mocks):

• user merging with success counts and cost
• org-scoping of the user groupBy queries (asserts organizationId in the where clause)
• spend-descending sort
• empty-user case (existing sort test)

Validation results

• API tests: 54 suites, 366 passed (364 pre-existing + 2 new)
• Web tests: 29 files, 147 passed
• npm run typecheck: clean
• npm run build: clean
• Diff: 3 files, 163 additions / 4 deletions; zero new dependencies; zero migrations; zero UI/deployment/CI changes

Risk

LOW. Additive read-only field on an existing org-scoped endpoint; same aggregation patterns and indexes as the existing breakdowns. No tenant-isolation change (still filtered by the authenticated organizationId).

Rollback

Revert the PR. The requestsByUser field disappears; nothing else changes. No migration or data rollback.

Production behavior

Not changed. No AI calls, providers, billing, auth, or RBAC behavior changed. Only a read-only response field was added.

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