Signals' metrics by person and whose key paid (#278)

With the server paying for everyone's Signals, its bill had no names.
ipx_signals_judged_total, ipx_signals_hits_total and ipx_jev_tokens_total
now carry user (the account's name) and key (server or own); a call
shared on the server's key is split by each person's share of its
questions. Icon picks bill the person on the server's key, key checks on
their own. The dashboard has tokens and items by person and a Who used
what table over its range.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/signals.md

@@ -392,7 +392,12 @@ erDiagram
 - **Keywords changed while an item is being judged** can leave that one item's verdict from the old
   words.
 - **The meter is the server's**, `ipx_jev_tokens_total` and `ipx_jev_requests_total` on
-  `/metrics`, and the Signals row of the dashboard: tokens as TypeSafe's answers count them. It is
+  `/metrics`, and the Signals row of the dashboard: tokens as TypeSafe's answers count them, by
+  person (`user`, the account's name) and whose key paid (`key`, `server` or `own`; #278), with
+  `ipx_signals_judged_total` and `ipx_signals_hits_total` the same way. A call shared by several
+  people on the server's key is split by each one's share of its questions. Icon picks bill the
+  person on the server's key, key checks the person on their own. The dashboard's **Who used what**
+  sums them over its time range. It is
   one call per item in the person's feeds as they come, and per item in the window for a new Signal.
 - **Only the window** is judged by publish date, apart from what an edit asks again.