Harden admin, ingest, and serving-runtime stalls

GET settings/effective no longer serializes the host Config (session_secret,
oauth secret, static tokens). Settings and policy writes require an admin
principal; upstream.token_env is host-only. Receive-pack rejects injected
ref names and oids. LFS PUT streams with a byte cap and validates the oid
before lfs_key. mode=none is refused unless listen is loopback.
accel-redirect is honoured only for loopback peers.

Ref-only writers remmap refs without loading pack indexes. gix fault rounds
use refresh_async. Remote pack indexes open on the bulk runtime.