ui(fairness): publish the automation-rate series with methodology note

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Problem

Once computed (sibling sub-issue), the automation-rate series needs a public surface. /fairness is the natural home — it is already the page outsiders are pointed to.

Scope

• Trend chart + current value on /fairness, clearly labeled with the definition and the backfill horizon
• Wire into the same basis/coverage annotations the other fairness numbers use (post-ui(public): wire fleetAccuracy.basis into the homepage and /fairness #9673 conventions)
• Include in the verify walkthrough's cross-checks (verifier compares endpoint vs page)

Acceptance

The number, its trend, and its definition are visible on /fairness without reading source.

[linear-code]

JSO-6787