Simple presence tracking system for PhoenixKit admin interface.

This is a lightweight alternative to Phoenix.Presence that works without requiring a full OTP application supervision tree.

Keys and multi-tab tracking

An authenticated visitor is tracked under user_key(user_uuid, session_id) ("user:<uuid>:<session_id>"), an anonymous one under "anonymous:<session_id>" — one ETS row per (identity, session), not per process. session_id is a stable per-login identifier that is the same across every LiveView mount sharing one login or one anonymous browser session, so opening a second tab, or mounting a second LiveView under the same session, tracks the SAME row instead of creating a second one: each track_* call adds its calling process's monitor to that row's monitor set rather than replacing the previous tab's. The row is deleted — and a *_session_disconnected event broadcast — only once its LAST monitor goes down; closing one tab while another is still open just shrinks the monitor set.

For an authenticated visitor tracked through PhoenixKitWeb.Users.Auth's on_mount hooks, session_id is a SHA-256 digest of the session token (Base.url_encode64/2, not the raw token or the raw live_socket_id itself) — stable per login for idempotent keying, but not reversible if it ends up somewhere it shouldn't (a log line, a PubSub event payload). A call site that still passes a raw session["live_socket_id"] (PhoenixKitWeb.Live.Dashboard.Overview's own local tracking call, which predates this scheme, and phoenix_kit_entities's own hook, a separate package) produces a DIFFERENT session_id for the same login until it is updated to match — a second, otherwise-idempotent row rather than a merge.

connected_at is pinned to the row's first appearance: a second tab merges its metadata into the existing row without resetting the timestamp.

This table is local to the BEAM node it runs on — presence does not merge across nodes in a multi-node deployment, so a visitor connected to node A is invisible to a "Live sessions" page served from node B.