Remove dead cache_ttl_seconds from THROTTLE_THRESHOLDS

[mrtwebdesign]

Source: PR #4 follow-up review (should address)

Problem

cache_ttl_seconds is defined in THROTTLE_THRESHOLDS (line 74) but never read anywhere in the code. The actual TTL calculation in persist_throttle_state() (line 527) uses max(60, level_min_dwell_seconds * 4).

Since THROTTLE_THRESHOLDS is exposed to operators via the hypercart_query_guard_load_thresholds filter, having a dead key is confusing — someone will try to tune it and wonder why nothing changes.

Fix

Either:

1. Remove it from the constant and document the actual TTL formula
2. Wire it up — replace the hardcoded formula in persist_throttle_state() with $thresholds['cache_ttl_seconds']

Option 1 is simpler unless there's a reason operators should control the cache TTL independently of dwell time.