Add query optimization and reliability improvements

- Consolidate StatsModel queries from 12 to 3 using conditional aggregation
- Add input validation to DashboardController (sort columns, dates, priorities)
- Combine getCategories/getTypes into single query
- Add transaction support to BulkOperationsModel with atomic mode option
- Add depth limit (20) to dependency cycle detection to prevent DoS
- Add caching to UserModel.getAllGroups() with 5-minute TTL
- Improve ticket ID generation with 50 attempts, exponential backoff, and fallback

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/UserModel.php

@@ -271,9 +271,20 @@ class UserModel {
      * Get all distinct groups from all users
      * Used for visibility group selection UI
      *
+     * Results are cached for 5 minutes to reduce database load
+     * since group changes are infrequent.
+     *
      * @return array Array of unique group names
      */
     public function getAllGroups(): array {
+        $cacheKey = 'all_groups';
+
+        // Check cache first
+        $cached = self::getCached($cacheKey);
+        if ($cached !== null) {
+            return $cached;
+        }
+
         $stmt = $this->conn->prepare("SELECT DISTINCT groups FROM users WHERE groups IS NOT NULL AND groups != ''");
         $stmt->execute();
         $result = $stmt->get_result();
@@ -289,6 +300,18 @@ class UserModel {
         // Return unique groups sorted alphabetically
         $uniqueGroups = array_unique($allGroups);
         sort($uniqueGroups);
+
+        // Cache the result
+        self::setCache($cacheKey, $uniqueGroups);
+
         return $uniqueGroups;
     }
+
+    /**
+     * Invalidate the groups cache
+     * Call this when user groups are modified
+     */
+    public static function invalidateGroupsCache(): void {
+        unset(self::$userCache['all_groups']);
+    }
 }