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/IValidationRule.cs

@@ -0,0 +1,23 @@
+namespace Core.Validation;
+
+/// <summary>
+/// Interface for all validation rules
+/// </summary>
+/// <typeparam name="TEntity">Type of entity being validated (Student, Team, StudentEventStatistics, etc.)</typeparam>
+public interface IValidationRule<TEntity>
+{
+    /// <summary>
+    /// Execute the validation rule against an entity
+    /// </summary>
+    /// <param name="entity">Entity to validate</param>
+    /// <param name="config">Validation configuration</param>
+    /// <returns>Validation warning if rule is violated, null otherwise</returns>
+    ValidationWarning? Validate(TEntity entity, ValidationConfiguration config);
+
+    /// <summary>
+    /// Determines if this rule applies to the given context
+    /// </summary>
+    /// <param name="context">Validation context</param>
+    /// <returns>True if the rule should be executed in this context</returns>
+    bool AppliesTo(ValidationContext context);
+}