internal/chat: messages fan out over NATS with a 7-day JetStream window (replay on join), keyed by project code so a channel is shared across nodes. chat.history/chat.post ops (post is not lease-gated — it mutates no store); web page with on-load history, a plain post form (no-JS baseline), and an SSE append stream (Datastar enhancement). Verified live: form post + cross-connection SSE delivery in-browser, and cross-node fan-out on a shared external NATS. Durable forum-thread archival waits on zit forum verbs (zit#76); the window is interim durability. check-in: 5670d3643d user: dmestas tags: trunk