Add admin endpoint to seed sample markets in non-prod environments

[greatest0fallt1me]

Description

This is a backend issue for the GrantFox campaign. Provide POST /api/admin/seed in dev/staging that inserts a small batch of sample markets for E2E and demos.

Requirements and Context

• Only in non-prod
• Idempotent
• Tracks seeded markets
• Docs explain
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b feature/admin-seed

2. Implement changes
   • src/routes/admin/seed.ts (new)
   • src/services/seedService.ts (new)
   • docs/seed.md (new)
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

feat: non-prod sample market seed endpoint

Acceptance Criteria

• Non-prod only
• Idempotent
• Tracked
• Doc shipped

Guidelines

• Minimum 90% test coverage on changed lines
• Input validation at the boundary; standardized error envelope
• Structured logging with correlation IDs
• Clear documentation and inline comments
• Timeframe: 96 hours