[Backend] seed/data/users.ts and remittance.ts are never invoked by an npm script

[grantfox-oss]

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Why this matters
backend/src/seed/index.ts imports backend/src/seed/data/users.ts and backend/src/seed/data/remittance.ts to build demo data, but package.json has no seed script, so new contributors have no documented way to populate a local database for manual testing.

Acceptance criteria

• Add a seed script to backend/package.json that runs backend/src/seed/index.ts
• Document the command in backend/README.md
• Confirm the script runs cleanly against a freshly migrated local database

Files to touch

• backend/package.json
• backend/src/seed/index.ts
• backend/README.md

Out of scope
Changing the shape of the seed data itself.