Get effective rate limits + override row for a tenant (admin only)

Body for GET /v1/admin/tenants/{tenant_id}/rate-limits.

Carries:

• defaults — the server-wide :class:Settings values, so the admin UI can render placeholders like "Default: 2/s" alongside each input.
• effective — what the limiter would actually use right now (defaults merged with the cached override, per-field).
• override — the raw override row (or None if no overrides set).
• ceilings — the validation upper bounds so the UI can surface them next to inputs without hard-coding the numbers.