refactor(credentials): consolidate Atlassian SA creation into /api/credentials

Atlassian service-account creation lived in its own route, contract, and
mutation hook, copy-pasting ~140 lines of insert/membership/audit/posthog
boilerplate from /api/credentials. Two endpoints means two authz paths,
two audit shapes, two TOCTOU stories — they will drift.

Fold Atlassian into the existing service_account branch of /api/credentials,
dispatching by providerId. The Atlassian validator (tenant_info + Bearer
/myself, SSRF host allowlist, typed error codes) lives in
lib/credentials/atlassian-service-account.ts and is the only Atlassian-
specific piece left. AtlassianValidationError maps to a {code, error} 400
in the existing catch block; the rest of the flow (transaction, members,
audit, posthog, dup-check) is now shared with Google SA + env credentials.

Delete:
- /api/auth/atlassian-service-account route
- contracts/atlassian-service-account.ts + barrel export
- useCreateAtlassianServiceAccount hook
- API audit baseline 727 → 726

Both forms (Google JSON-key, Atlassian token+domain) now call
useCreateWorkspaceCredential with the appropriate body shape.

Author: TheodoreSpeaks