Resolve invoice and backend integration issues (#684-#687)

[JhayJ22]

Summary

• Expose the invoice creation counter as a read-only contract view and document it in the ABI.
• Port API integration coverage into the canonical contracts workspace, including the current treasury settlement lifecycle.
• Add backend ESLint configuration and CI wiring, replacing production-source any casts where encountered.
• Add GET /invoices/:id/events backed by indexed event data, ordered by ledger, with documented response and missing-invoice behavior.
• Update CI to test the pull request's in-tree contracts workspace.

Validation

• git diff --check passed.
• Builds and tests were not run, as requested.
• ESLint was attempted; existing backend parse errors and numerous test-fixture any usages remain and need follow-up before the lint job is green.

Closes #684
Closes #685
Closes #686
Closes #687

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