Add migration: make stripe_charge_id nullable, add manual_payment_id to earlybird purchases (#788)

## Summary

- Makes the `stripe_charge_id` column on `kiloclaw_earlybird_purchases`
nullable (was `NOT NULL`)
- Adds a new nullable `manual_payment_id` column (`text`, `UNIQUE`) to
support non-Stripe payment tracking

## Details

This migration allows earlybird purchases to be recorded via manual
payment in addition to Stripe. Both `stripe_charge_id` and
`manual_payment_id` are nullable with unique constraints, so each
purchase can be identified by either payment method.

### Files changed
- `packages/db/src/schema.ts` — updated Drizzle schema definition
- `packages/db/src/migrations/0041_earlybird_manual_payment.sql` —
migration SQL
- `packages/db/src/migrations/meta/0041_snapshot.json` — Drizzle
snapshot
- `packages/db/src/migrations/meta/_journal.json` — journal entry for
migration 0041

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Author: olearycrew

packages/db/src/migrations/0041_earlybird_manual_payment.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "kiloclaw_earlybird_purchases" ALTER COLUMN "stripe_charge_id" DROP NOT NULL;--> statement-breakpoint
+ALTER TABLE "kiloclaw_earlybird_purchases" ADD COLUMN "manual_payment_id" text;--> statement-breakpoint
+ALTER TABLE "kiloclaw_earlybird_purchases" ADD CONSTRAINT "kiloclaw_earlybird_purchases_manual_payment_id_unique" UNIQUE("manual_payment_id");