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/CcsModels.cs

@@ -349,6 +349,8 @@ public class CcsUserRating
     public string UserId { get; set; } = "";
     public int Elo { get; set; } = 1000;
     public int RatedCaseCount { get; set; }
+    public int? HistoryBaselineElo { get; set; }
+    public DateTime? HistoryStartedUtc { get; set; }
     public DateTime UpdatedUtc { get; set; } = DateTime.UtcNow;
 }