Dedup identical opus-tier rate rows in modelPricing.js EXACT_RATES

[atomantic]

In server/lib/modelPricing.js, all five claude-opus-* rows in EXACT_RATES (~lines 39-43) carry an identical [5.0, 25.0] rate pair:

'claude-opus-5': [5.0, 25.0],
'claude-opus-4-8': [5.0, 25.0],
'claude-opus-4-7': [5.0, 25.0],
'claude-opus-4-6': [5.0, 25.0],
'claude-opus-4-5': [5.0, 25.0],

Confirmed still the case. Because of this, the { test: /opus/i, rateModel: 'claude-opus-5' } line in FAMILY_RULES (~line 97) must be re-pointed at every model bump purely for zero behavioral effect — it only relabels the reported rateModel for unrecognized opus ids to whichever pointer key currently has the rates, since every opus row already resolves to the same numbers.

Fix shape

Either:

1. Extract a shared const OPUS_TIER_RATES = [5.0, 25.0] and reference it from every claude-opus-* row plus the FAMILY_RULES fallback, so the pointer bump becomes unnecessary (the fallback can point at the constant directly instead of a specific model id), or
2. Let a family rule yield rates directly ({ test: /opus/i, rates: [5.0, 25.0] }) instead of pointing at a peer EXACT_RATES key.

Either retires the per-bump FAMILY_RULES edit that migration 206 (and every opus bump before it) had to make for no behavioral change.

Low priority — listed for completeness, not urgency. Surfaced alongside the migration-skeleton factoring item on the opus-5 PR.

Migrated from PLAN.md by /do:replan --issues.

[atomantic]

Picked fix shape 1 (shared rate constant, family-rule pointer derived rather than hand-written) over shape 2 (a family rule yielding rates inline). Recording the reasoning since the issue left the choice open.

Shape 2 cannot preserve behavior, because rateModel is load-bearing beyond the label:

• client/src/pages/UsagePage.jsx renders Priced as ${m.rateModel} in the per-model cost tooltip.
• attributedModel() in server/services/usageReconciler.js compares the resolved rateModel of a recorded launch model and a transcript model to decide whether they are the same family, and deliberately treats null as "unrecognized, therefore not a match".

An inline-rates rule has to report either rateModel: null — which breaks that comparison outright — or a synthetic label such as claude-opus, which would no longer equal the exact table's label, so opus and claude-opus-5 would stop reconciling as one model.

Shape 1 keeps the pointer a real EXACT_RATES key while still retiring the per-bump edit: OPUS_MODEL_IDS is ordered newest-first, generates the tier's rows, and its head feeds the /opus/i rule. An opus bump is now a one-line prepend, and the pointer can no longer go stale or dangle since it is derived rather than typed. Behavior is unchanged — resolveModelRates and isFreeModelId return identical results across 954 provider/model pairs before and after.

Shipped in #4259.