feat(database): add prisma migrations, deploy scripts, and seed data

[Samaro1]

Adds Prisma migration tooling under packages/database/migrations/
Introduces initial PostgreSQL schema for core entities (merchant, users, customer, payment intent, escrow, subscription, invoice, treasury, payout, webhook, audit log)
Adds production migration deploy script and test migration script
Adds development seed data and environment example
Changes
Added Prisma schema and migration SQL:
packages/database/migrations/schema.prisma
packages/database/migrations/migrations/202603240001_initial_schema/migration.sql
Added scripts:
packages/database/migrations/deploy.sh
packages/database/migrations/test-migrate.sh
package scripts in packages/database/package.json
Added seed data:
packages/database/migrations/seed.js
Added env template:
packages/database/.env.example
Validation
Ran migration deploy and seed successfully against a PostgreSQL test instance using db:test:migrate.

Closes #94

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[MissBlue00]

@Samaro1 the ci/cd check failed and there are merge conflicts, please resolve

[Samaro1]

@MissBlue00 resolved and fixed the CI tests, could you please make the CI tests automated so it happens once a PR has been made?

[Samaro1]

@MissBlue00 can you please review and merge