infra(ci): re-add cargo fmt/clippy gates and raise backend coverage to 60%

[devonahi]

• Re-added rustfmt and clippy components to contracts CI job.
• Restored backend coverage thresholds to 60% and added 17 new test files to exceed the goal (achieved ~72% on core files).
• Removed dead unauthenticated top-level stream routes.
• Removed unused sorobanWithdraw import in stream controller.
• Fixed small TS error in soroban-worker helpers test.

Summary of changes:

1. CI Infrastructure: Re-added cargo fmt --check and clippy quality gates to
   the contracts job in .github/workflows/ci.yml. The Rust toolchain now
   correctly includes rustfmt and clippy components.
2. Backend Coverage:
   • Raised backend test coverage to over 60% (achieving ~72% on core
     application files by excluding generated code and external RPC
     wrappers).
   • Restored coverage thresholds in backend/vitest.config.ts to 60% across
     the board.
   • Added 17 new test files covering controllers, middleware, services, and
     repositories to ensure robust validation.
3. Code Cleanup:
   • Deleted the dead, unauthenticated backend/src/routes/stream.routes.ts
     file.
   • Removed the unused sorobanWithdraw import from
     Final CI Verification (Local):

• Contracts: cargo fmt and clippy pass.
• Backend Build: npm run build (tsc) passes.
• Backend Tests: All 160+ tests pass, and coverage meets the restored 60%
  threshold.

Closes #523
Closes #533
Closes #536
Closes #537