Add contract, revenue split, and vesting workflows

[k-deejah]

Summary

• Add an observable, retry-aware contract upgrade proposal workflow that validates and simulates upgrades, resolves organization signers, and sends idempotent notifications.
• Add revenue split recipient-update and distribution workflows with share/amount validation, simulation, wallet-signed transaction staging, confirmation, and signed-envelope redaction.
• Add an automatic due-vesting notification scheduler and an organization-scoped notification workflow.
• Extend durable workflow kinds with migration 062 and a guarded rollback.

Verification

• backend: npm run test:workflows passes (55 tests); Postgres integration tests are skipped locally without WORKFLOW_TEST_DATABASE_URL and run in CI with Postgres.
• Prettier and changed-file diagnostics pass.
• Backend-wide npm run build remains blocked by pre-existing errors in src/index.ts, fxRateService.test.ts, and sorobanEventIndexer.ts; the migration dry-run is blocked by the current pg ESM import issue.

Closes #1240
Closes #1241
Closes #1242
Closes #1243

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