Fixes
- IMAP: stop the startup connection stampede behind Gmail "Connection not available" errors (#384). With several accounts on one provider (especially Gmail), startup opened too many connections at once — persistent connects, the body-fetch pool, and folder backfills all firing together — which tripped the provider's connection limit until backfills subsided, leaving one account repeatedly failing to connect. All connection types now share a single per-host admission gate that caps concurrent connection establishment, smoothing the burst without limiting how many connections stay open.
- The IPv4 fallback added in 3.1.1 (#382) no longer retries when the provider is refusing connections (a connection limit, not a broken-IPv6 stall) — that was doubling connect times under load.
Upgrade: GHCR/image users pull 3.1.2; source builds pull main.