feat(cache-proposals): cost-weighted threshold optimization

[jamby77]

Stacked on #224 (confidence-score).

Summary

• Records cost_saved_micros on similarity-window entries in the semantic cache, with retroactive population from later store() calls.
• Feeds cost_saved_micros from similarity samples into the cache-proposals threshold recommendation engine.
• Tier-gated cost-weighted threshold decisions (enterprise only).
• Cost-aware reasoning text when cost-weighting fires; cost response fields scoped to the active direction.
• Cleanup: zscore lookup, symmetric loosen cost fields, denominator note.

Closes the "Cost-Weighted Threshold Optimization" card on the BetterDB project board.

Test plan

• semantic-cache cost-instrumentation tests pass
• cache-proposals threshold recommendation tests cover cost-weighting path
• enterprise tier gate verified (non-enterprise unaffected)
• integration: end-to-end cost flow from store() → similarity sample → recommendation

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Note

Medium Risk
Changes threshold tuning behavior for enterprise customers and adds best-effort Valkey bookkeeping on the hot check/store path; failures are swallowed but could skew cost metrics early in rollout.

Overview
Semantic cache now attaches cost_saved_micros to rolling similarity-window events (hits from entry cost_micros, misses as null), tracks pending misses in a __miss_pending zset keyed by prompt hash, and backfills miss rows when a later store() supplies token cost—so threshold analytics can see dollar impact on near-misses, not only on hits.

Cache proposals reads that field from the window, and on enterprise license only switches uncertain-hit / near-miss tighten and loosen decisions to cost-weighted rates (with optional USD fields and cost-aware reasoning); Pro and below keep count-based logic.

New unit tests cover instrumentation, pending-miss lifecycle, and tier-gated recommendations.

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