Upsert per-tenant rate-limit overrides (admin only)

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

Every numeric field is independently optional: omit a field (or send null) to inherit the default for that knob. Sending all four fields as null upserts a row with no overrides — equivalent to leaving the row absent and relying on defaults, but keeps the audit columns (notes / updated_by / updated_at) populated.

Server enforces > 0 and the ceilings declared above; values outside that band cause a 400, not a silent clamp, so admins notice a typo instead of getting a quietly wrong limit.

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.