fix: Restore idempotency for recommendation report jobs

**Problem:** Previous commit stubbed HasReportBeenSentAsync/MarkReportSentAsync due to Dapper AOT error, but didn't restore idempotency check/mark calls. This broke CLAUDE.md guarantee: "Each job must be replayable without side effects."

**Solution:** Implement idempotency using proven ADO pattern from GetSellDecisionsAsync:
- HasReportBeenSentAsync: SELECT COUNT from recommendation_sent_log
- MarkReportSentAsync: CREATE TABLE IF NOT EXISTS + INSERT with ON CONFLICT

**Changes:**
- RecommendationReportGenerator: Restored real idempotency logic (ADO pattern, no Dapper)
- GenerateDailyRecommendationJob: Restore idempotency check/mark calls
- GenerateWeeklyRecommendationJob: Restore idempotency check/mark calls
- GenerateMonthlyRecommendationJob: Restore idempotency check/mark calls

**Guarantees Restored:**
- Partial failure safe (Telegram succeeds, job throws → no duplicate on retry)
- Manual trigger safe (dashboard re-run → skips if already sent)
- [DisableConcurrentExecution] per CLAUDE.md blocking rule

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Host/Jobs/GenerateDailyRecommendationJob.cs

@@ -38,8 +38,16 @@ public sealed class GenerateDailyRecommendationJob
             var now = _clock.UtcNow;
             var reportDate = now.DateTime.Date;
 
+            var idempotencyKey = $"daily:{reportDate:yyyy-MM-dd}";
+            if (await reportGenerator.HasReportBeenSentAsync(idempotencyKey, cancellationToken))
+            {
+                _logger.LogInformation("Daily report already sent for {Date}. Skipping", reportDate);
+                return;
+            }
+
             var report = await reportGenerator.GenerateDailyRecommendationAsync(reportDate, cancellationToken);
             await reportGenerator.SendRecommendationReportAsync(report, cancellationToken);
+            await reportGenerator.MarkReportSentAsync(idempotencyKey, cancellationToken);
 
             _logger.LogInformation(
                 "Daily recommendation report sent. Date={Date}, Recommendations={Count}",

src/KArtSell.Host/Jobs/GenerateMonthlyRecommendationJob.cs

@@ -41,8 +41,16 @@ public sealed class GenerateMonthlyRecommendationJob
             // Get start of current month
             var monthStart = new DateTime(currentDate.Year, currentDate.Month, 1);
 
+            var idempotencyKey = $"monthly:{monthStart:yyyy-MM-dd}";
+            if (await reportGenerator.HasReportBeenSentAsync(idempotencyKey, cancellationToken))
+            {
+                _logger.LogInformation("Monthly report already sent for {Month}. Skipping", monthStart.ToString("yyyy-MM"));
+                return;
+            }
+
             var report = await reportGenerator.GenerateMonthlyRecommendationAsync(monthStart, cancellationToken);
             await reportGenerator.SendRecommendationReportAsync(report, cancellationToken);
+            await reportGenerator.MarkReportSentAsync(idempotencyKey, cancellationToken);
 
             _logger.LogInformation(
                 "Monthly recommendation report sent. Month={Month}, Recommendations={Count}",

src/KArtSell.Host/Jobs/GenerateWeeklyRecommendationJob.cs

@@ -44,8 +44,16 @@ public sealed class GenerateWeeklyRecommendationJob
                 daysToSubtract += 7;
             var weekStart = currentDate.AddDays(-daysToSubtract);
 
+            var idempotencyKey = $"weekly:{weekStart:yyyy-MM-dd}";
+            if (await reportGenerator.HasReportBeenSentAsync(idempotencyKey, cancellationToken))
+            {
+                _logger.LogInformation("Weekly report already sent for week starting {Date}. Skipping", weekStart);
+                return;
+            }
+
             var report = await reportGenerator.GenerateWeeklyRecommendationAsync(weekStart, cancellationToken);
             await reportGenerator.SendRecommendationReportAsync(report, cancellationToken);
+            await reportGenerator.MarkReportSentAsync(idempotencyKey, cancellationToken);
 
             _logger.LogInformation(
                 "Weekly recommendation report sent. WeekStart={Date}, Recommendations={Count}",

src/KArtSell.Host/Jobs/RecommendationReportGenerator.cs

@@ -101,14 +101,47 @@ public sealed class RecommendationReportGenerator
         await SendTelegramMessageAsync(message, cancellationToken);
     }
 
-    public Task<bool> HasReportBeenSentAsync(string idempotencyKey, CancellationToken cancellationToken)
+    public async Task<bool> HasReportBeenSentAsync(string idempotencyKey, CancellationToken cancellationToken)
     {
-        return Task.FromResult(false);
+        const string sql = @"
+            SELECT COUNT(1)
+            FROM recommendation_sent_log
+            WHERE idempotency_key = $1";
+
+        await using var connection = await _connectionFactory.OpenAsync(cancellationToken);
+        await using var command = connection.CreateCommand();
+        command.CommandText = sql;
+        command.Parameters.Add(command.CreateParameter());
+        command.Parameters[0].Value = idempotencyKey;
+
+        var result = await command.ExecuteScalarAsync(cancellationToken);
+        return (long?)result > 0;
     }
 
-    public Task MarkReportSentAsync(string idempotencyKey, CancellationToken cancellationToken)
+    public async Task MarkReportSentAsync(string idempotencyKey, CancellationToken cancellationToken)
     {
-        return Task.CompletedTask;
+        const string createTableSql = @"
+            CREATE TABLE IF NOT EXISTS recommendation_sent_log (
+                id BIGSERIAL PRIMARY KEY,
+                idempotency_key TEXT NOT NULL UNIQUE,
+                sent_at TIMESTAMP NOT NULL DEFAULT NOW()
+            )";
+
+        await using var connection = await _connectionFactory.OpenAsync(cancellationToken);
+        await using var createCmd = connection.CreateCommand();
+        createCmd.CommandText = createTableSql;
+        await createCmd.ExecuteNonQueryAsync(cancellationToken);
+
+        const string insertSql = @"
+            INSERT INTO recommendation_sent_log (idempotency_key, sent_at)
+            VALUES ($1, NOW())
+            ON CONFLICT (idempotency_key) DO NOTHING";
+
+        await using var insertCmd = connection.CreateCommand();
+        insertCmd.CommandText = insertSql;
+        insertCmd.Parameters.Add(insertCmd.CreateParameter());
+        insertCmd.Parameters[0].Value = idempotencyKey;
+        await insertCmd.ExecuteNonQueryAsync(cancellationToken);
     }
 
     private async Task<IReadOnlyList<SellRecommendation>> GetSellDecisionsAsync(