One ballot per matchup + rating-aware matchmaking; register Opus 4.8 as pending

Every (voter, case, pair) now gets exactly one vote: the client sampler
only draws matchups the browser hasn't voted on (with a graduation state
once all are spent, counter shows voted/total pairs), and the worker
normalizes pair order on insert under a unique (ip, case, pair) index so
side-swapping can't double-count. Daily IP cap drops 300 -> 200 since a
voter graduates at ~171 pairs today.

Matchmaking is now active rather than uniform: unvoted pairs are drawn
with weight closeness/(1+battles) — close ratings and under-played
matchups surface more, unrated combos get placement matches first — with
a 25% pure-exploration floor. Ratings come from the community pool (60s
cached) and fall back to local votes. Rankings stay Bradley-Terry.

Also registers the empty outputs/claude-opus-4-8/ folder as a pending
registry entry.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

models/claude-opus-4-8.json

@@ -0,0 +1,5 @@
+{
+  "label": "Claude Opus 4.8",
+  "vendor": "Anthropic",
+  "order": 15
+}