feat(jwt-claims): add jwt_private.current_api_id()

[pyramation]

Summary

Track A of constructive-io/constructive-planning#1149 (API invocation provenance). Adds jwt_private.current_api_id() to the jwt-claims package, following the exact pattern of jwt_private.current_database_id():

CREATE FUNCTION jwt_private.current_api_id() RETURNS uuid
-- reads current_setting('jwt.claims.api_id', TRUE)
-- returns NULL when the claim is absent or not a valid uuid (NOTICE on bad uuid)
-- STABLE

Deploy/revert/verify triple + pgpm.plan entry; regenerated sql/pgpm-jwt-claims--0.15.5.sql at the current extension default_version (same convention as the current_role_type addition — no version bump; lerna handles npm versioning at release time).

Consumed by constructive-io/constructive-db#2129, where the function_callable_check policy's api arm requires a function_api_bindings row with api_id = jwt_private.current_api_id(). The vendored copy in constructive-db pgpm-modules/jwt-claims is updated identically in that PR.

Link to Devin session: https://app.devin.ai/sessions/c57a4982235d4b4098e9a49bbe99c355
Requested by: @pyramation

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