feat(analytics): add privacy-safe product usage event table

[JSONbored]

Parent phase: #237
Parent roadmap: #127

Problem

Current code has AI usage events and analytics UI, but v1 adoption needs a durable product-usage event table for app/MCP/GitHub App/extension/Raycast/PWA activation without leaking private scoreability or identity-sensitive details.

Acceptance criteria

• Schema/migration adds privacy-safe product usage events with role, surface, event name, outcome, coarse target, and created-at fields.
• Event payloads avoid raw token, wallet/hotkey, local path, private scoreability, and raw prompt/source content leakage.
• Repository layer exposes safe write/read helpers with redaction tests.

Validation expected

• Migration and persistence tests cover event insertion and redaction.
• API tests cover aggregate reads without exposing raw private payloads.

Public-output safety criteria

• Public text is sanitized before reaching GitHub comments, issue bodies, PR bodies, extension-visible public panels, or copied public snippets.
• Tests cover forbidden wallet/hotkey, reward-estimate, trust-score, public-score-prediction, private-reviewability, private-scoreability, and farming-language leakage.

Cross-cutting acceptance criteria

• Preserve the repo quality gate: npm run test:ci, 97%+ global coverage, and the local branch coverage target for touched code.
• Keep public/private boundaries explicit. Public GitHub output must not expose wallets, hotkeys, reward estimates, raw trust scores, public score predictions, private reviewability, private scoreability context, or farming language.
• Add/update focused tests for the changed behavior instead of relying on green checks alone.