FlashLin 3.1.0: RealWeb realtime hub

RealWeb realtime hub ([realweb])

• New built-in realtime hub. Browsers and apps connect to FlashLin over a
  WebSocket with subprotocol realweb.v1; FlashLin terminates the connection
  itself, so backends need no socket code. Guide: docs/REALWEB.md.
• Security: HS256 JWT tokens (standard, mint with any JWT library) sent in the
  first frame, never the URL; exp required, only HS256 accepted, 30 s skew.
  Origin checked at the handshake (same host by default, or
  allowed_origins). Per-channel subscribe/publish permissions from the token;
  public_channels readable anonymously. Connections close when their token
  expires unless refreshed with {"t":"auth"}; a refresh can narrow rights.
• Channels multiplexed over one connection, with per-channel sequence
  numbers, in-memory history (history, history_ttl, 1 MiB per channel)
  and resume via since/epoch; gap: true tells a client it missed data.
• Acknowledged publishes, presence (count, users), heartbeats in both
  directions.
• Limits: max_connections, max_message_size, max_channels,
  messages_per_second (refuses, then closes floods), and a bounded
  send_queue that disconnects stalled consumers. Abusive connections are
  closed with a lingering close so clients receive the close reason.
• Fan-out serializes each message once and shares it among subscriber queues;
  queued messages are written in batches with one flush.
• POST /flashlin/realweb/publish and GET /flashlin/realweb loopback
  control endpoints (same protection as /flashlin/apps), RealWeb counters
  on /metrics, and CLI flashlin realweb token | publish | status.
• Dependency-free JS client served at <path>/client.js: reconnect with
  backoff and jitter, resume without duplicates, token refresh, client-side
  heartbeat watchdog, queued publishes while offline.
• Example: examples/realweb.toml (chat page plus a supervised Node app that
  mints tokens and pushes a clock).
• Plain WebSocket proxying is unchanged.

Docker: docker pull endurasolution/flashlin:3.1.0 (also 3.1, 3, latest; linux/amd64)

Guide: docs/REALWEB.md