fix: correct README setup refs, align ARC explorer URL, and defer webhook Supabase init

[Sertug17]

Summary

This PR fixes onboarding/documentation inconsistencies and a runtime initialization issue:

• README
  • Updates clone URL from akelani-circle/fintech-starter to circlefin/arc-fintech
  • Updates cd path from fintech-starter to arc-fintech
  • Removes dangling your-ngrok-url ... from step 4 line
• ARC explorer consistency
  • Aligns lib/circle/gateway-sdk.ts explorer URL with lib/constants/block-explorers.ts (https://testnet.arcscan.app)
• Webhook route initialization
  • Defers Supabase admin client initialization to runtime helper instead of eager module level initialization
• DX
  • Adds explicit placeholder test script

Why

• README had stale setup instructions.
• ARC explorer URL was inconsistent across files.
• Eager module-level Supabase client init can fail when env values are absent during import/build contexts.

Validation notes

• Branch pushed successfully from fork:
  • fix/readme-explorer-webhook-init
• Local environment had tooling/env constraints affecting full lint/build verification, but the scoped fixes are applied and isolated.

[Sertug17]

@circlefin/maintainers quick ping 🙏
All requested fixes are included in this PR:

• README clone/cd corrections
• dangling ngrok reference removal
• ARC explorer URL alignment
• deferred Supabase webhook client init
• explicit test script placeholder

Happy to adjust quickly if you prefer test-script removal from this PR scope.

[Sertug17]

On reflection, closing this in favor of #9 by @skyc1e which already
addresses the README clone URL, ngrok reference, and the
gateway-sdk.ts explorer URL those three fixes overlap entirely.

The other two changes in this PR (deferred Supabase init, test
script placeholder) deserve their own focused PRs with proper
testing if they're useful, rather than being bundled in.

Apologies for the duplication, @skyc1e your PR is the right one.
Will keep an eye on #9 and open separate, well-scoped PRs in the
future.