docs(api): document registerDomain's idempotent reclaim behavior

[AmirF194]

Summary

registerDomain has always returned 201 with the existing row when a caller re-registers a domain their own account already owns (#819's plan-cap exemption depends on this). That behavior was undocumented everywhere except a code comment, and one design doc stated the opposite: that a retry always 409s. This PR documents the real behavior and corrects the false premise.

Client surface checklist

• OpenAPI spec + generated types refreshed (make spec then make generate-sdk)
• TypeScript SDK regenerated (docstring only, no behavior change)
• Python SDK regenerated (docstring only, no behavior change)
• Everything else in the template checklist doesn't apply: no handler, migration, CLI, or MCP change.

What changed

• Added a Description to the registerDomain operation (internal/httpapi/domains.go), matching its listDomains/deleteDomain/verifyDomain siblings, which already had one.
• Regenerated api/openapi.yaml and both generated SDK bases from it.
• Added one sentence to docs/api.md's domains section.
• Corrected docs/design/api-v1-redesign.md, which twice said a register_domain retry is "a 409, never a duplicate." It reclaims the existing row with 201.

Explicitly not changing

The 201 status code itself stays as is. /v1 is frozen at v1.5.0, so 201 -> 200 would be a breaking response-success-status-removed change under the compat gate, and the Python SDK's generated client only deserializes '201' for this operation, so 200 would silently return None to callers. This PR is documentation only.

Verification

• go test ./internal/httpapi/... green, including the two existing tests that already pin this behavior (TestRegisterDomainAtCapAllowsReclaimOfOwnedDomain, TestRegisterDomainAtCapStillReturnsConflictForAnotherAccountsDomain).
• make spec-check and make openapi-compat-check (against upstream main) both clean: no drift, no breaking changes.
• make generate-sdk-check's normalization tests and scripts/check-sdk-operation-coverage.py both pass; the only generated-code diff is the new description string in the domains API docstrings.
• go build ./..., go vet ./internal/httpapi/..., and gofmt are clean. Did not run the full DB-backed integration/e2e suites, since nothing here touches runtime behavior.

Fixes #824