feat: implement rate limiting

[tim-hm]

Problem

With NUCs now supported client-side (browsers), nildb is now more exposed to high-volume traffic. Thus, we need to implement fair-use rate limiting to protect nodes from abuse and ensure service stability.

Proposed Solution

• Start with an off the shelf rate limiting implementation like hono-rate-limiter
• Apply limits both on both per-builder account and per IP
• The rate limits (e.g., requests per second, burst capacity) should be configurable at the node level.
• When a client exceeds the rate limit, the API should return a standard ⁠429 Too Many Requests HTTP status code.

Acceptance criteria

• Requests from a single IP exceeding the configured rate are rejected with a ⁠429 error.
• An aggregate of requests using the same builder did and exceeding the configured rate are rejected with a ⁠429 error.
• The rate-limiting logic is enabled by default but can be configured or disabled via node settings.

• The impact on performance for non-rate-limited requests is negligible.