v1.2.1

Harden SDK against full live-API audit (v1.2.1)

Audited file-by-file and re-verified end-to-end against the live API
(sandbox.assinafy.com.br/v1).

Fixes
- Remove Webhooks.DeleteAsync: DELETE /webhooks/subscriptions does not exist
  (404 route). Use InactivateAsync / UpdateSubscriptionAsync(IsActive=false).
- Fix GetSigningProgressAsync/IsFullySignedAsync to fall back to per-signer
  completed flags when an assignment has no summary (previously reported 0%).

Features / gaps
- Add signing-order Step to SignerRef and TemplateSigner.
- UploadAndRequestSignaturesAsync accepts Method + per-signer
  verification/notification/step.

Robustness / security
- Attach auth per request instead of mutating the shared HttpClient headers;
  reject ApiKey+Token together; DI recycles via PooledConnectionLifetime.
- FindByEmailAsync pages through all results; SignMultiple/DeclineMultiple
  reject empty lists; WaitUntilReadyAsync tolerates a transient post-create 404;
  UpdateSubscriptionAsync guards null events; numeric->string keeps exact text.

Cleanup
- Remove dead Assignment.Expiration and redundant signer_ids payload field;
  DocumentDetails/TemplateDetails and pages share base records; seal exceptions.

Tests / docs
- 92 unit tests (+18 regression) and a gated LiveIntegrationTests suite;
  README + CHANGELOG; deterministic builds; publish.yml version-stamp fix.