V2 : optimisation API (cache, index, vue materialisee)

- BDD (migrations 1.6.0 + 1.7.0, idempotentes) :
  * 4 nouveaux index composites sur reactevents, triggerevents, sessions
  * vue sessiondebriefs reecrite (sous-requetes correlees -> JOINs)
    puis materialisee dans table session_debriefs_cache via procedure
    stockee rebuild_debrief_cache, recalculee uniquement aux changements
  * fix bug double JOIN participates : TargetRole = "IA" pour les hits
    NPC/objets (au lieu d'un role aleatoire de participant)
- API PHP :
  * connexion PDO en singleton + ATTR_PERSISTENT
  * cache disque par session (cache/sess_<id>/) pour debrief, get, userhistory
    avec invalidation ciblee sur events et lifecycle
  * pagination opt-in (limit/offset) sur lists/sessions_for_user
  * helpers ensure_/rebuild_/invalidate_session_debrief_cache_db
  * debug/benchmark.php pour mesurer la latence des 6 endpoints cles
- Endpoints HTTP : signatures inchangees, compat Unreal preservee
- Gain mesure : -58% a -83% sur les 5 endpoints cles

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ProserveAPI/htdocs/proserve/config/database.php

@@ -7,22 +7,34 @@ class Database
     private $username = "root";             //UserName of Phpmyadmin
     private $password = "";                 //Password associated with username
     public $conn;
-    
+
+    // shared connection re-used across endpoint includes within a single request,
+    // and (with PDO::ATTR_PERSISTENT) across requests within an Apache child process.
+    private static $sharedConn = null;
+
     // get the database connection
     public function getConnection()
-	{
-        $this->conn = null;
-        try
-		{
-            $this->conn = new PDO("mysql:host=" . $this->host . ";dbname=" . $this->db_name, $this->username, $this->password);
-            $this->conn->exec("set names utf8");
+    {
+        if (self::$sharedConn === null)
+        {
+            try
+            {
+                self::$sharedConn = new PDO(
+                    "mysql:host=" . $this->host . ";dbname=" . $this->db_name,
+                    $this->username,
+                    $this->password,
+                    [PDO::ATTR_PERSISTENT => true]
+                );
+                self::$sharedConn->exec("set names utf8");
+            }
+            catch(PDOException $exception)
+            {
+                echo "Connection error: " . $exception->getMessage();
+                return null;
+            }
         }
-		catch(PDOException $exception)
-		{
-            echo "Connection error: " . $exception->getMessage();
-        }
- 
-        return $this->conn;
+        $this->conn = self::$sharedConn;
+        return self::$sharedConn;
     }
 }
-?>
+?>