Add isolated question completion credits

api/QuestionsApi/Services/CreditStore.cs

@@ -0,0 +1,78 @@
+using MongoDB.Bson;
+using MongoDB.Driver;
+using QuestionsApi.Models;
+
+namespace QuestionsApi.Services;
+
+public sealed class CreditStore
+{
+    private readonly IMongoCollection<UserCreditAccount> _accounts;
+    private readonly TimeProvider _clock;
+
+    public CreditStore(MongoDatabaseProvider mongo, TimeProvider clock)
+    {
+        _accounts = mongo.Database.GetCollection<UserCreditAccount>("UserCreditAccounts");
+        _clock = clock;
+        _accounts.Indexes.CreateOne(new CreateIndexModel<UserCreditAccount>(
+            Builders<UserCreditAccount>.IndexKeys.Ascending(value => value.UserId),
+            new CreateIndexOptions { Unique = true, Name = "uq_credit_user" }));
+    }
+
+    public async Task<bool> AwardForFirstAnswerAsync(string userId, string attemptId)
+    {
+        if (string.IsNullOrWhiteSpace(attemptId)) return false;
+        var now = _clock.GetUtcNow().UtcDateTime;
+        await EnsureAndCleanAsync(userId, now);
+
+        var filters = Builders<UserCreditAccount>.Filter;
+        var belowCap = new BsonDocumentFilterDefinition<UserCreditAccount>(
+            new BsonDocument("$expr", new BsonDocument("$lt", new BsonArray
+            {
+                new BsonDocument("$size", "$RecentEarnings"),
+                CreditEconomy.RollingEarningLimit
+            })));
+        var filter = filters.Eq(value => value.UserId, userId) &
+            filters.Not(filters.ElemMatch(value => value.RecentEarnings,
+                earning => earning.OperationId == attemptId)) &
+            belowCap;
+        var update = Builders<UserCreditAccount>.Update
+            .Inc(value => value.Credits, 1)
+            .Push(value => value.RecentEarnings, new CreditEvent
+            {
+                OperationId = attemptId,
+                CreatedUtc = now
+            })
+            .Set(value => value.UpdatedUtc, now);
+        return (await _accounts.UpdateOneAsync(filter, update)).ModifiedCount == 1;
+    }
+
+    public async Task<CreditStatusResponse> GetStatusAsync(string userId)
+    {
+        var now = _clock.GetUtcNow().UtcDateTime;
+        await EnsureAndCleanAsync(userId, now);
+        var account = await _accounts.Find(value => value.UserId == userId).FirstAsync();
+        return new CreditStatusResponse
+        {
+            Credits = account.Credits,
+            EarnedInRollingWindow = account.RecentEarnings.Count,
+            RemainingEarningCapacity = Math.Max(
+                0, CreditEconomy.RollingEarningLimit - account.RecentEarnings.Count)
+        };
+    }
+
+    private async Task EnsureAndCleanAsync(string userId, DateTime now)
+    {
+        await _accounts.UpdateOneAsync(
+            value => value.UserId == userId,
+            Builders<UserCreditAccount>.Update
+                .SetOnInsert(value => value.UserId, userId)
+                .SetOnInsert(value => value.UpdatedUtc, now),
+            new UpdateOptions { IsUpsert = true });
+        var cutoff = now - CreditEconomy.EarningWindow;
+        await _accounts.UpdateOneAsync(
+            value => value.UserId == userId,
+            Builders<UserCreditAccount>.Update.PullFilter(
+                value => value.RecentEarnings,
+                earning => earning.CreatedUtc <= cutoff));
+    }
+}

api/QuestionsApi/Services/UserProfileStore.cs

@@ -19,7 +19,8 @@ public sealed class UserProfileStore
         "Step3Outcomes", "QuestionReports", "CcsCaseReports", "AppFeedback", "UserStudyProfiles",
         "ExternalIdentities", "ExternalAuthTransactions",
         "FlashcardDecks", "FlashcardNotes", "FlashcardSchedules", "FlashcardReviewLogs",
-        "FlashcardMediaAssets", "FlashcardSettings", "FlashcardImportPreviews"
+        "FlashcardMediaAssets", "FlashcardSettings", "FlashcardImportPreviews",
+        "UserCreditAccounts"
     ];
 
     public UserProfileStore(MongoDatabaseProvider mongo)