Feat: add rating

migrations/002_add_stats_elo.down.sql

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+DROP TRIGGER IF EXISTS update_player_rating;
+
+ALTER TABLE DROP COLUMN rating;

migrations/002_add_stats_elo.up.sql

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+ALTER TABLE player_stats 
+ADD COLUMN rating REAL NOT NULL DEFAULT 1000.0;
+
+CREATE TRIGGER update_player_rating 
+BEFORE UPDATE OF games_played, games_won ON player_stats
+WHEN NEW.games_played = OLD.games_played + 1
+BEGIN
+    UPDATE player_stats
+    SET rating = OLD.rating + 
+                 32.0 * (
+                     CASE 
+                         WHEN NEW.games_won = OLD.games_won + 1 
+                             THEN 1.0 - 0.5  -- Win term: 0.5
+                         ELSE 0.0 - 0.5      -- Loss term: -0.5
+                     END
+                 ) + 
+                 0.05 * (1000.0 - OLD.rating)
+    WHERE id = OLD.id;
+END;
+