Add comprehensive validation system with runtime configuration

Implements a flexible validation framework for student rankings, team assignments, and team composition with administrator-configurable rules and thresholds.

Validation System:
- Reflection-based rule discovery eliminates manual registration
- Base classes (RequiredEventTypeRuleBase, EventCountThresholdRuleBase) reduce code duplication by ~250 lines
- 12 validation rules covering event requirements, counts, and team constraints
- Configurable severity levels (Warning/Error) per rule type
- ValidationService with caching for optimal performance
- Rules apply contextually (StudentRanking, StudentAssignment, TeamComposition)

Configuration & Admin UI:
- ValidationSettings admin page for editing thresholds and severity levels
- ChapterSettings admin page for editing chapter information
- Settings stored in Data/appsettings.json for runtime configuration
- JSON config works in both development and production environments
- Auto-creates Data directory and config template on first run

User Experience:
- ValidationWarnings component displays inline warnings/errors
- Integrated in Event Ranking, Registration, and Team Assignment pages
- Color-coded severity indicators (warning yellow, error red)
- Includes "Too Many Regional Events" rule (max 3 recommended)

Technical Improvements:
- Template Method pattern for rule base classes
- Singleton rule instances with lazy initialization
- Configuration loaded via IConfiguration with fallback to defaults
- Safe JSON updates preserve other appsettings sections

Core/Validation/Rules/StudentAssignmentRules/TooFewEventsRule.cs

@@ -0,0 +1,36 @@
+using Core.Entities;
+using Core.Validation.Rules.BaseRules;
+
+namespace Core.Validation.Rules.StudentAssignmentRules;
+
+/// <summary>
+/// Validation rule that checks if a student has too few event assignments
+/// </summary>
+public class TooFewEventsRule : EventCountThresholdRuleBase
+{
+    protected override int GetCount(StudentEventStatistics statistics) => statistics.EventCount;
+
+    protected override int GetThreshold(ValidationConfiguration config) => config.MinRecommendedEvents;
+
+    protected override bool ViolatesThreshold(int count, int threshold) => count < threshold;
+
+    protected override ValidationSeverity GetBaseSeverity(ValidationConfiguration config) => config.EventCountSeverity;
+
+    protected override int? GetCriticalThreshold(ValidationConfiguration config) => config.MinCriticalEvents;
+
+    protected override bool ViolatesCriticalThreshold(int count, int criticalThreshold) => count < criticalThreshold;
+
+    protected override string Code => "TOO_FEW_EVENTS";
+
+    protected override string GetMessage(int count, int threshold) =>
+        $"Student has {count} events (min recommended: {threshold})";
+
+    protected override Dictionary<string, object> BuildAdditionalMetadata(StudentEventStatistics statistics, ValidationConfiguration config)
+    {
+        return new Dictionary<string, object>
+        {
+            { "MinRecommended", config.MinRecommendedEvents },
+            { "MinCritical", config.MinCriticalEvents }
+        };
+    }
+}