feat(api): maintainer quality-dashboard service + endpoint

[JSONbored]

Parent roadmap: #525
Parent phase: #528

Background

The maintainer quality dashboard (#539) is a UI; its data service + API are non-visual and reusable. Carving them out lets contributors build the backend while the rendering stays owner-led.

Goal

Build the maintainer-quality-dashboard service + the GET /v1/app/maintainer-dashboard enrichment that #539 renders.

Current Behavior

operator-dashboard.ts exists but surfaces install counts / recommendation quality, not slop/duplicate/gate-outcome trends or contributor quality bands.

Desired Behavior

• New src/services/maintainer-quality-dashboard.ts mirroring operator-dashboard.ts: read+shape existing snapshots (buildQueueTrendReport, weekly-value-report.ts, collision clusters, gate-outcome audit) into queue-health, slop/duplicate trends, gate outcomes, and top contributors by quality band.
• Enrich the maintainer-dashboard API; show generatedAt + stale flag; don't recompute heavy signals on load.

Implementation Requirements

• Pure service + API; no chart/UI. Bands not raw numbers.

Public/Private Output Boundaries

• Maintainer/operator-authenticated; never raw credibility/reward/wallet fields.

Acceptance Criteria

• The endpoint returns shaped trend/outcome data scoped to the maintainer's repos.

Testing Requirements

• npm run test:ci, 97%+ coverage. Shaping + access-scope + redaction tests.

Wave 3 extension (#525)

• Triage → action queue (service/logic, non-visual): assemble a ranked "review now / redirect duplicate / close as slop / merge-ready" list — the maintainer mirror of the contributor decision-pack — from burden forecast + collision clusters + outcome patterns + the gate verdict + feat(github-app): merge-readiness aggregate gate #551 merge-readiness, scoped to the maintainer's repos. feat(ui): maintainer repo/org quality dashboard #539 renders it; unified miner+maintainer cockpit (dual-role home) #700 (dual-role cockpit) consumes it. Public-safe, bands not raw numbers.