Validate campaign milestone target amounts

[Errordog2]

Summary

• Require milestone targetAmount in the create campaign DTO and validate it as a positive numeric string.
• Replace the implicit (m.targetAmount ?? 0) as any fallback with a service-level guard that rejects missing, non-numeric, negative, zero, and sub-minimum milestone targets.
• Add CampaignsService regression coverage for invalid milestone targets and the valid minimum amount path.

Closes #12

Validation

• npm test -- campaigns.service.spec.ts passed (7 tests).
• npm run build passed.
• git diff --check -- src/campaigns/dto/create-campaign.dto.ts src/campaigns/campaigns.service.ts src/campaigns/campaigns.service.spec.ts passed.
• Sensitive scan of changed files returned no account/payment/private-key markers.

Notes

• Ran npm install; Prisma client generation completed successfully. Existing npm audit findings are dependency-tree issues outside this validation fix.

[ibrahimmosouf-png]

Clean validation — the regex + min threshold cover the Soroban precision edge case nicely. Thanks for tightening this up ✅

[Errordog2]

Thanks for merging. Is there any additional GrantFox/reward tracking step needed from my side for #12?