Paginate the leaderboard from materialized player ratings

Store each player's leaderboard totals (wins, accumulated points and the
latest display name) on player_rating alongside the Elo rating, updated
in the same transaction as the match, so the board is a single indexed
table query instead of aggregating every participation on each request.

GET /api/leaderboard is now page-number paginated (page/limit) and
returns the page, page count and total, plus the caller's own rank and
page when authenticated; focus=me returns the caller's page directly.
The migration adds the columns, backfills them from the match history
(inserting rows for players from pre-Elo matches at the initial rating)
and replaces the index. backfill_elo rebuilds the aggregates too.

The web page opens on the page containing the signed-in player, marks
their row and adds a numbered footer pager.

server/packages/tavolo-platform/README.md

@@ -33,15 +33,18 @@ itself ships no game.
   revalidate the token, so stale or duplicate deliveries are harmless.
 - `models.py` — `Match` (`game_type`, timestamps, game-specific JSON
   `result`), `MatchPlayer` (seat, team, won, points, Elo delta, JSON
-  `details`) and `PlayerRating` (Elo per `(user_sub, game_type)`).
+  `details`) and `PlayerRating` (Elo and leaderboard aggregates per
+  `(user_sub, game_type)`).
 - `stats.py` — `save_match_result` (engine `MatchResult` → Postgres,
-  transactionally, with Elo) and `apply_elo`.
+  transactionally, with Elo and the leaderboard aggregates) and
+  `apply_match_stats`.
 - `elo.py` — chess-style Elo math generalized to two-team matches.
-- `backfill_elo.py` — rebuild every rating from the match history:
-  `python -m tavolo.platform.backfill_elo --database-url postgres://…`.
+- `backfill_elo.py` — rebuild every rating and aggregate from the match
+  history: `python -m tavolo.platform.backfill_elo --database-url postgres://…`.
 - `auth.py`, `http.py`, `pagination.py`, `openapi.py`,
-  `tortoise_mixin.py` — OIDC helpers, JSON helpers, keyset pagination,
-  shared OpenAPI fragments, the TortoiseORM lifecycle mixin.
+  `tortoise_mixin.py` — OIDC helpers, JSON helpers, keyset and
+  page-number pagination, shared OpenAPI fragments, the TortoiseORM
+  lifecycle mixin.
 
 ## Adding a game