Add rating snapshots, percentile, and exam readiness to the dashboard API

Backend half of the dashboard stat-card redesign. Three new data
points the old DashboardResponse had no way to compute:

- Week-over-week rating deltas: UserRating/CcsUserRating only ever
  stored the current Elo (mutated in place, UpdatedUtc just means
  "last touched"), so there was no way to know what a rating was 7
  days ago. Added UserRatingSnapshot (new collection, unique on
  UserId+DateUtc) and UserRatingSnapshotService, a BackgroundService
  following AnswerStatisticsMaintenanceService's exact shape (a
  PeriodicTimer plus a Mongo lease so replicas behind the HPA don't
  race), writing one snapshot per user per day. No historical
  backfill - deltas are simply null for a user's first ~7 days after
  this ships, which the DTO represents honestly via nullable ints
  rather than a fake "+0".
- Percentile rank: DashboardStore now counts how many users have a
  higher OverallElo/Elo than the current user out of the total.
- Exam Readiness + projected pass rate: reuses the existing, tested
  IRT-based ExamSimulationScore.PassProbability (a real normal-CDF
  estimate of scoring >=200, computed in ExamSimulationScoring.Calculate)
  rather than inventing a new prediction model. DashboardStore pulls
  the latest completed simulation's score directly and maps
  PassProbability to a High/Medium/Low label via tunable thresholds.
- Average Score's "+6% vs last 100" needed no new data source at all -
  just one more slice (Skip(100).Take(100)) over the QuestionAttempts
  already loaded for FirstAttemptAccuracy.

Added DashboardStore.Percentile/ReadinessLabel as pure static helpers
(matching the existing Percentage/CalculateCurrentStreak pattern) and
unit tests for both. Full backend suite: 165 passed, 0 failed.

api/QuestionsApi/Models/DashboardModels.cs

@@ -10,6 +10,13 @@ public sealed class DashboardResponse
     public int CcsCasesCompleted { get; set; }
     public int FirstAttemptAccuracy { get; set; }
     public int AccuracySampleSize { get; set; }
+    public int? AccuracyDeltaVsPreviousSample { get; set; }
+    public int? QuestionBankRatingDeltaThisWeek { get; set; }
+    public int? QuestionBankPercentile { get; set; }
+    public int? CcsRatingDeltaThisWeek { get; set; }
+    public int? CcsPercentile { get; set; }
+    public string? ExamReadinessLabel { get; set; }
+    public int? ExamReadinessProjectedPassPercent { get; set; }
     public int QuestionsThisWeek { get; set; }
     public int StudyTimeSecondsThisWeek { get; set; }
     public int CurrentStreak { get; set; }
@@ -33,6 +40,7 @@ public sealed class DashboardTrendPoint
     public int Total { get; set; }
     public int Accuracy { get; set; }
     public int? QuestionBankRating { get; set; }
+    public int? CcsRating { get; set; }
 }
 
 public sealed class DashboardSystemPerformance