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/ValidationContext.cs

@@ -0,0 +1,27 @@
+namespace Core.Validation;
+
+/// <summary>
+/// Context in which validation is being performed
+/// </summary>
+public enum ValidationContext
+{
+    /// <summary>
+    /// Validating a single student's event rankings
+    /// </summary>
+    StudentRanking = 0,
+
+    /// <summary>
+    /// Validating a single team configuration
+    /// </summary>
+    Team = 1,
+
+    /// <summary>
+    /// Validating a student's assigned events
+    /// </summary>
+    StudentAssignment = 2,
+
+    /// <summary>
+    /// Validating student registration data
+    /// </summary>
+    StudentRegistration = 3
+}