docs: update API rate limits

[samrathreddy]

Summary

• update the public rate-limits page to match the current backend request-rate defaults
• distinguish request-rate 429 responses from credit/spend-cap 402 responses
• update synthetic data, training job, and API key endpoint pages so the inline limit notes match the consolidated table

Validation

• git diff --check
• stale-value search for old documented limits
• mint dev --no-open served the updated rate-limits page at http://localhost:3001/api-reference/rate-limits
• mint validate runs under Node 22, but fails on existing site config issues: favicon generation treats the remote DuckDuckGo favicon as a local path, and strict validation fails on an existing OpenAPI warning from docs.json.

[jaanavit]

Thanks for surfacing this — two of the facts here are real and currently missing from main, verified directly against the Brain source:

1. POST /create-api-key is rate-limited to 50/hour — confirmed at brain/api/api_keys_router.py:103, @limiter.limit("50/hour", key_func=generation_user_key). Not documented anywhere on the site today.
2. POST /generate has a second, global limit on top of the per-user one — 500/min shared across all callers platform-wide, not just your own 120/min budget. Confirmed at brain/api/felix_generate_router.py:90-95, @limiter.shared_limit(generation_global_limit, scope=GENERATION_GLOBAL_SCOPE, ...). api-reference/synthetic-data.mdx and guides/synthetic-data.mdx still only mention the 120/min per-user figure with no hint of the shared ceiling.

The rest of this PR's rate-limits.mdx changes are based on an older state of the page (the 1,000/min defaults, the daily_spend_cap_exceeded header framing) that's since been superseded by #47 — main now has the corrected numbers plus the actual 402/403 credit-cap mechanism, so merging this as-is would regress that. Given the conflicts GitHub is already flagging, we're going to close this rather than rebase it, but I'll open a follow-up that folds in the two real findings above (the create-api-key limit + the generate global cap) on top of the current page. Appreciate the catch on those two.