Churn Cohort Builder

Assembles the retention cohort from Zoho Billing reports. Two types:

Pre-churn — non-renewing subscriptions (still active, scheduled to cancel at cycle end).
Post-churn — recently cancelled subscriptions (last 7 days, win-back candidates).

Output schema

Field	Source
`subscription_id`, `customer_id`, `customer_name`	report row
`plan_code`, `plan_name`	report row
`mrr`	`recurring_amount` normalized to monthly (annual ÷ 12, quarterly ÷ 3)
`currency`, `country`	report row; `"Unknown"` if absent
`cancel_reason_raw`	free-text from report — classifier buckets it later
`cancellation_date`	scheduled end date (pre-churn) or actual cancel date (post-churn)
`ltv`	report field; default `0` + flag `ltv_unavailable: true` if missing
`ltd`	report field (days); compute from `activated_at` if absent; `0` + `ltd_unavailable: true` if both absent
`cohort`	`"pre_churn"` or `"post_churn"`