feat: add CSV import for student event rankings

Let advisors load preference ranks from a converted CSV with fuzzy matching, known aliases, and a parse-preview-save page instead of relying on the ranking editor.

Co-authored-by: Cursor <cursoragent@cursor.com>

WebApp/Services/IStudentEventRankingSaveService.cs

@@ -0,0 +1,34 @@
+using Core.Models;
+
+namespace WebApp.Services;
+
+/// <summary>
+/// Persists parsed student event rankings.
+/// </summary>
+public interface IStudentEventRankingSaveService
+{
+    /// <summary>
+    /// Students in the parse result who already have rankings stored.
+    /// </summary>
+    Task<IReadOnlyList<string>> GetStudentsWithExistingRankingsAsync(
+        StudentEventRankingParseResult parseResult,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Replaces rankings for students who have at least one accepted rank in the parse result.
+    /// Other students are left unchanged.
+    /// </summary>
+    Task<StudentEventRankingSaveResult> SaveAsync(
+        StudentEventRankingParseResult parseResult,
+        CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// Outcome of saving imported event rankings.
+/// </summary>
+public class StudentEventRankingSaveResult
+{
+    public int StudentsUpdated { get; set; }
+
+    public int RankingsSaved { get; set; }
+}