refactor(semantic-cache): drop redundant B3 read-time threshold layer

[jamby77]

Summary

PR #134 landed an earlier B3 commit (5s-TTL read-time HGETALL on every check()) and #148's commit added a 30s background refresh that mutates defaultThreshold / categoryThresholds in place. Both read the same {prefix}:__config hash; running both is duplicated work and the file even ended up with a duplicate private readonly configKey: string field declaration.

This PR keeps the 30s background-refresh approach (cleaner lifecycle, opt-out flag, Prometheus counter, no per-call overhead) and removes the B3 machinery.

Changes

• Removes private fields thresholdOverrides, thresholdOverridesCachedAt, thresholdOverridesRefresh and the THRESHOLD_OVERRIDES_TTL_MS constant.
• Removes private helpers resolveThreshold, getThresholdOverrides, refreshThresholdOverrides.
• Restores check() / checkBatch() threshold resolution to the simple options.threshold > categoryThresholds[category] > defaultThreshold chain; refreshConfig() updates those mutable fields.
• Deletes runtime-threshold-overrides.test.ts (covered the deleted helpers).
• Removes the duplicate configKey field declaration and constructor assignment.
• CHANGELOG: drops the read-time-overrides bullet, expands the periodic-refresh bullet to spell out hash field semantics and the synchronous-first-tick guarantee, and rewords the Behavior change note.

Net diff: −315 / +21 LoC.

Trade-off

Propagation latency for an approved threshold proposal goes from ~5s (B3 read-time) to ~30s worst-case (configurable). Acceptable given the human-approval flow upstream.

Test plan

• pnpm --filter @betterdb/semantic-cache exec vitest run — 128/128 pass
• pnpm --filter @betterdb/semantic-cache exec tsc --noEmit

---

Note

Medium Risk
Changes how check()/checkBatch() resolve thresholds by removing the 5s TTL per-call __config reads, so runtime threshold updates now depend solely on the periodic refresh cadence and could affect hit/miss behavior if refresh is delayed or disabled.

Overview
Removes the redundant read-time {prefix}:__config threshold override layer (cached for 5s) and the associated helpers/state, so check() and checkBatch() now resolve thresholds only as options.threshold → categoryThresholds[category] → defaultThreshold.

Keeps the background config refresh mechanism as the single source of runtime threshold updates, cleans up the duplicate configKey wiring, and deletes the unit test suite that covered the removed per-call override behavior. Documentation in CHANGELOG.md is updated to describe the periodic refresh semantics (including synchronous first refresh on initialize()) and the __config hash field mapping.

^{Reviewed by Cursor Bugbot for commit 2979891. Bugbot is set up for automated code reviews on this repo. Configure here.}