feat(scripts): add migrate-contribution-amount

scripts/README.md

@@ -49,3 +49,26 @@ If something goes wrong mid-run, the backup JSON contains the pre-migration stat
 - Identify which members had `helcimSubscriptionId` before it was unset.
 
 Backup files are gitignored (`.migration-backup-*.json`).
+
+---
+
+## migrate-contribution-amount.cjs
+
+One-time migration: converts `Member.contributionTier` (String enum) to
+`Member.contributionAmount` (Number). Idempotent — only touches documents that
+still have the legacy field.
+
+### Usage
+
+```bash
+node scripts/migrate-contribution-amount.cjs           # dry-run
+node scripts/migrate-contribution-amount.cjs --apply   # write
+```
+
+### What it does
+
+Iterates members with `contributionTier` still set, parses each value as an integer
+(valid: `"0"`, `"5"`, `"15"`, `"30"`, `"50"`), and writes the converted value to
+`contributionAmount`. Unsets the old field. Skips any record that cannot be parsed.
+
+Safe to re-run: only matches members where `contributionTier` still exists.