feat: churn-analysis golden fixtures and deterministic tests

[iexwr]

Summary

Closes #886 — expand the churn-analysis script test suite with checked-in golden fixtures and a fixture-driven runner, so analytics metrics can no longer regress silently.

Changes

• scripts/data/churn-analysis-fixtures.json (new): 7 golden fixtures covering:
  • Cohort retention under both new and retention resubscription logic (80%+ of the metric surface).
  • Merchant-level churn breakdown with single-subscriber merchant filtering.
  • Multi-completed-cohort churn projection (average churn + projected count).
  • Grace-period cancelled_with_refund treated identically to cancelled.
  • Empty event list and single-event edge cases.
  • Single-subscriber merchant exclusion to avoid statistical skew.
• scripts/test-churn-analysis.ts (rewritten): loads fixtures from data/, asserts cohort metrics, merchant breakdown, and projection per fixture per logic; keeps the existing report-structure and SQLite/RPC fallback coverage; exits non-zero on any assertion failure.
• scripts/package.json: adds npm test → tsx test-churn-analysis.ts.

Validation

cd scripts && npm install && npm test   # all assertions PASS, exit 0

Acceptance Criteria

• Tests cover core metrics (cohort retention 30d/90d, merchant churn, projection)
• Fixtures checked in under scripts/data/ with documented descriptions
• Non-zero exit on assertion failure (process.exit(1))

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[iexwr]

Rebased onto the latest master (which had advanced with #980/#986, touching scripts/package.json). Resolved the conflict by keeping both the new test:event-dedup/test:renewal-forecast scripts and the test (churn-analysis) script, and fixed a missing comma the merge had left in package.json (valid JSON verified). npm test passes.

Note: npm run typecheck currently fails on upstream master itself (errors in pre-upgrade-check.ts, rotate-fee-collector.ts, etc. from the recently merged PRs) — this is pre-existing and unrelated to this PR; test-churn-analysis.ts is typecheck-clean.