docs: document quality_tier (A/B/C) on /opportunities/ev

[Mlaz-code]

Summary

Documents the quality_tier field (grader-certified A/B/C), which went live on /opportunities/ev on 2026-07-14 when EV_QUALITY_TIER was flipped on in production (sharp-api-go #1204). The field was shipping undocumented — this closes that gap.

What changed

• content/en/api-reference/opportunities-ev.mdx — quality_tier row in the response-field table + a new Quality Tier section (A/B/C criteria table, output-only note, and a real high-EV-but-B example that preempts the "why isn't my high-EV row an A?" support ticket).
• content/en/concepts/ev-calculation.mdx — a Quality Tiers methodology section explaining the grading rationale (why stated EV and realized results diverge; the low-band bleed; segment reliability caps).
• public/openapi.json — EVOpportunity.quality_tier (type: string, enum: [A, B, C], not in required); info.version 3.1.0 → 3.2.0.
• public/openapi-version.json + CHANGELOG.md — version bump + entry.

Verified against the live API (docs-reality)

• Output-only — there is no quality_tier query filter (confirmed in the Go handler); documented as such, no invented parameter.
• The wire ev_percentage is the exact value the tier bands use (transformEV passes it through), so the criteria reference the visible field — no phantom "corrected EV".
• Omitted on suspended rows (is_suspended) — matches transformEV deleting the key; schema leaves it out of required.
• The A-vs-B example (bwin baseball 1st_5_innings_total_runs @ 7.35 = A vs total_runs @ 7.94 = B) is a real captured pair — the higher-EV row is B because its segment is watch-flagged.

Scope

English source + the language-agnostic OpenAPI spec. de / pt-BR / es prose translations are a follow-up (tracked separately; the spec already covers machine consumers for all locales). No nav/_meta.js change (edits to existing pages).

Checks

• OpenAPI version gate (scripts/check-openapi-version.mjs) passes locally: info.version 3.1.0 -> 3.2.0; OK.
• JSON valid; <Callout> tags balanced; all cross-link anchors resolve. Vercel preview will validate the full MDX build.

Refs sharp-api-go#1204 · crew #2950

Type: docs

[Mlaz-code]

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