SSE hint stream

`text/event-stream` of **hints, not transports**: the client refetches via REST (conditional, ETag) on every hint and on every (re)connect, so missed hints are harmless by construction. Auth via query parameters (browser `EventSource` cannot set headers). Server requirement (tested invariant, not a habit): `subscriber_hash` is scrubbed from access/proxy log lines for this endpoint — query-string credentials otherwise leak into logs. Events (`id:` on every event is an opaque resume token): * `hint` — something changed for this subscriber; refetch list/counts. Debounced server-side (at most one per subscriber per interval). Keep-alive is a comment frame (`: ping`) every 30 seconds, not an event — comment frames are deliberately invisible to EventSource listeners. Unknown future event types must be ignored by clients. Resume: browsers replay `Last-Event-ID` automatically on reconnect; the server answers by emitting an immediate `hint` if anything changed after that token (it does not replay individual missed events — the REST refetch is the recovery mechanism). On graceful shutdown the server sends a `retry:` directive with jitter before closing, so a deploy does not produce a reconnect stampede.

curl -X GET "https://example.com/v1/inbox/stream"

"id: 01h455vb4pex5vsknk084sn02q\nevent: hint\ndata: {\"reason\":\"notification\"}\n"

{
  "error": {
    "code": "invalid_request",
    "message": "string"
  }
}