Move API limiter setup out of internal/api and into a dedicated
internal/api/apilimiter package, then wire it into serve-time config
reloads so rate limit changes are picked up without restarting the
service.

This change replaces the old LimiterOptions type with an apilimiter.Limiter
instance passed through api.WithLimiter(...). API construction now defaults
to apilimiter.New(...) when no limiter is injected, and tests are updated
to use the new option-based wiring.

The new apilimiter package centralizes:
- construction of all ratelimit and tollbooth limiters
- mapping between config/env vars and limiter fields
- copy/update logic for reusing existing limiter state where possible
- structured logging for limiter changes during config reload

On config reload in serve():
- keep track of the previously active limiter set
- call previousLim.Update(...) against the latest config
- build the new API with the updated limiter set
- store the new API, reload apiworker config, and retain the latest limiter
  for the next reload cycle

This fixes the prior behavior where hot config reload rebuilt the API but
kept stale limiter settings, meaning rate-limit changes were not applied
until process restart.

Additional ratelimit changes:
- persist the original parsed conf.Rate value in conf.Rate via val
- add GetRateValue() for logging/comparison purposes
- extend ratelimit.Limiter with Config() so limiters can expose their
  backing configuration
- add ratelimit.Equal(...) helper to compare limiters, configs, and rate
  strings consistently
- store conf.Rate on BurstLimiter and IntervalLimiter and expose Config()
- add String() methods to identify limiter type in tests/debug output
- rename IntervalLimiter.limit to events for clarity

Behavioral note:
- BurstLimiter documentation now matches implementation for non-positive
  event counts: burst size becomes 0, so no events are allowed

Tests:
- update API tests to inject limiters through api.WithLimiter
- update options tests to validate apilimiter.New
- expand ratelimit tests to cover type identification and equality semantics
- add dedicated apilimiter tests that verify only the expected fields change
  when each config/env-backed limiter value is modified

Co-authored-by: fadymak <dev@fadymak.com>