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/tests/test_scopa.py

@@ -190,7 +190,12 @@ class ScopaWebSocketTest(unittest.TestCase):
                         self.assertEqual(1, game["last_move"]["seat"])
                         self.assertEqual(card, game["last_move"]["card"])
                         if options:
-                            self.assertEqual(2, game["players"][1]["captured_count"])
+                            # The pile grows by the captured table cards
+                            # plus the card just played.
+                            self.assertEqual(
+                                len(options[0]) + 1,
+                                game["players"][1]["captured_count"],
+                            )
                         else:
                             self.assertEqual(5, len(game["table"]))