Build invoice payment cohort analyzer

[Kingsman-99]

Label: complexity: high
Points: 200

Description

For creator analytics dashboards, group invoices by creation-week/month cohorts and compute funding-completion rates per cohort, to visualize trends over time.

Technical Context

New src/cohortAnalyzer.ts. Buckets Invoice[] by configurable time period (week/month) using Invoice creation timestamp data, computes completion rate per bucket.

Acceptance Criteria

• analyzeCohorts(invoices, period: "week"|"month") returns ordered cohort buckets with { period, total, completed, completionRate }
• Handles sparse data (some periods with zero invoices) without gaps breaking the ordering
• Pure function, no network calls
• Tests: weekly bucketing correctness; monthly bucketing correctness; sparse-period handling

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