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/tests/test_pagination.py

@@ -14,7 +14,14 @@ from datetime import datetime, timezone
 from typing import Any, List
 
 from tavolo.platform.models import Match
-from tavolo.platform.pagination import Cursor, decode_cursor, encode_cursor, paginate
+from tavolo.platform.pagination import (
+    Cursor,
+    PageParamsError,
+    decode_cursor,
+    encode_cursor,
+    paginate,
+    parse_page_params,
+)
 
 from helpers import async_test
 
@@ -77,5 +84,22 @@ class PaginateCursorTypesTest(unittest.TestCase):
         self.assertEqual(match_id, id_values[0])
 
 
+class PageParamsTest(unittest.TestCase):
+    def test_defaults(self) -> None:
+        self.assertEqual((1, 20), parse_page_params(""))
+
+    def test_page_and_limit(self) -> None:
+        self.assertEqual((3, 5), parse_page_params("page=3&limit=5"))
+
+    def test_limit_is_clamped(self) -> None:
+        self.assertEqual((1, 100), parse_page_params("limit=500"))
+        self.assertEqual((1, 1), parse_page_params("limit=0"))
+
+    def test_invalid_values_raise(self) -> None:
+        for query in ("page=0", "page=-2", "page=abc", "limit=abc"):
+            with self.assertRaises(PageParamsError, msg=query):
+                parse_page_params(query)
+
+
 if __name__ == "__main__":
     unittest.main()