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_stats.py

@@ -133,11 +133,25 @@ class ScoponeStatsTest(unittest.TestCase):
             self.assertIn("your_elo_delta", results[0])
             self.assertEqual(4, len(results[0]["players"]))
 
-            board = await client.get("/api/leaderboard")
+            with oidc_user("alice"):
+                board = await client.get("/api/leaderboard")
             self.assertEqual(200, board.status_code)
-            by_sub = {r["user_sub"]: r for r in board.json()["results"]}
+            body = board.json()
+            by_sub = {r["user_sub"]: r for r in body["results"]}
             self.assertEqual(4, len(by_sub))
             self.assertTrue(all(r["matches"] == 1 for r in by_sub.values()))
+            self.assertEqual(1, body["page"])
+            self.assertEqual(1, body["pages"])
+            self.assertEqual(4, body["total"])
+            # The caller's own rank rides along on the response.
+            self.assertIsNotNone(body["your_rank"])
+            self.assertEqual(1, body["your_page"])
+
+            paged = await client.get("/api/leaderboard?limit=1&page=2")
+            self.assertEqual(200, paged.status_code)
+            self.assertEqual(4, paged.json()["pages"])
+            self.assertEqual(2, paged.json()["page"])
+            self.assertEqual(1, len(paged.json()["results"]))
 
 
 async def _finished_briscola_session() -> GameSession: