fix: chess moves/resign silently failing — non-existent columns

The `matches` table has no `last_activity` or `ended_at` columns.
PostgREST rejects the entire PATCH when ANY column doesn't exist,
causing every move update and resignation to silently fail.

- Remove `last_activity` from handleGameMove (column doesn't exist)
- Change `ended_at` to `completed_at` in handleResign, handleDraw,
  handleComplete (correct column name per DB schema)

This was the root cause of zero sync in multiplayer chess.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api/game.php

@@ -109,7 +109,7 @@ function handleGameMove($db, string $userId, array $input): void {
     if (!$matchId) jsonError('match_id required');
 
     $sdb = supabaseService();
-    $update = ['updated_at' => date('c'), 'last_activity' => date('c')];
+    $update = ['updated_at' => date('c')];
 
     if (!empty($input['fen'])) $update['current_fen'] = $input['fen'];
     if (!empty($input['move'])) {
@@ -163,7 +163,8 @@ function handleResign($db, string $userId, array $input): void {
     $sdb->update('matches', [
         'status' => 'completed',
         'result' => $result,
-        'ended_at' => date('c')
+        'completed_at' => date('c'),
+        'updated_at' => date('c')
     ], ['id' => 'eq.' . $matchId]);
 
     jsonResponse(['result' => $result]);
@@ -182,7 +183,8 @@ function handleDraw($db, string $userId, array $input): void {
     $sdb->update('matches', [
         'status' => 'completed',
         'result' => 'draw',
-        'ended_at' => date('c'),
+        'completed_at' => date('c'),
+        'updated_at' => date('c'),
         'game_state' => ['draw_accepted' => true]
     ], ['id' => 'eq.' . $matchId]);
 
@@ -205,7 +207,8 @@ function handleComplete($db, string $userId, array $input): void {
         'result' => $result,
         'current_fen' => $fen,
         'pgn' => $pgn,
-        'ended_at' => date('c')
+        'completed_at' => date('c'),
+        'updated_at' => date('c')
     ], ['id' => 'eq.' . $matchId]);
 
     // Use universal rewards function
@@ -380,7 +383,7 @@ function handleChessLeave(string $userId, array $input): void {
 
     $sdb->update('matches', [
         'game_state' => $gameState,
-        'last_activity' => date('c')
+        'updated_at' => date('c')
     ], ['id' => 'eq.' . $matchId]);
 
     jsonResponse(['success' => true, 'bot_replaced' => true]);