fix(http): replace rate limiter semaphore with per-endpoint token bucket

[austin-barrington]

Description

The old Semaphore never refilled, permanently blocking requests after the initial budget was consumed. Use token buckets that refill every second, with independent budgets for /write and /query. Rate limiting now runs before auth and records hyperbytedb_rate_limit_denied_total.

Related issues

Closes #62, #80

Checklist

• cargo fmt --all --check passes
• cargo clippy --all-targets -- -D warnings passes
• cargo test --lib passes
• cargo test --test '*' passes
• New features have documentation (user-guide or developer-guide)
• New config keys have entries in docs/user-guide/configuration.md and config.toml.example
• New metrics follow the naming convention (hyperbytedb_*)
• Breaking changes are noted in the PR description

Release notes

Test plan