Proxying for axios relied on the per-instance httpAgent/httpsAgent, which axios.create() snapshots
from axios.defaults at construction time. API clients constructed at import (such as
BaseScanner.tmdb, avalabilitySync.tmdb) are built before createCustomProxyAgent runs, so they
snapshotted no proxy agent and bypassed the proxy permanently, while clients created per-request
afterwards were proxied. This fix resolves the agent per request from a live module-level holder via
a stable interceptor that is registered on the default instance and every ExternalAPI client, so
construction order no longer matters. forceIpv4First has been moved to the same path logic as well.

fix #3193