[AZ-1126] Migrate capturedAt to DateTimeOffset

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

SatelliteProvider.IntegrationTests/UavUploadValidationTests.cs

@@ -91,6 +91,9 @@ public static class UavUploadValidationTests
         // Rule 14: type mismatch (lat as string)
         await ItemLatTypeMismatch_Returns400(apiUrl, secret);
 
+        // AZ-1126: offset-less capturedAt rejected at deserializer
+        await ItemCapturedAtOffsetLess_Returns400(apiUrl, secret);
+
         Console.WriteLine("✓ UAV upload metadata validation tests: PASSED");
     }
 
@@ -429,6 +432,33 @@ public static class UavUploadValidationTests
         Console.WriteLine("  ✓ items[0].capturedAt = now-60d rejected with indexed errors path");
     }
 
+    private static async Task ItemCapturedAtOffsetLess_Returns400(string apiUrl, string secret)
+    {
+        Console.WriteLine();
+        Console.WriteLine("AZ-1126 AC-2: per-item capturedAt without explicit UTC offset → HTTP 400");
+
+        // Arrange
+        var coord = NextTestCoordinate();
+        var metadata = new
+        {
+            items = new[]
+            {
+                new { latitude = coord.Latitude, longitude = coord.Longitude, tileZoom = 18, tileSizeMeters = 200.0, capturedAt = "2026-06-26T12:00:00" },
+            },
+        };
+        using var client = CreateClientWithGpsToken(apiUrl, secret);
+
+        // Act
+        using var response = await PostBatch(client, metadata, new[] { CreateValidJpeg() });
+        var problem = await ProblemDetailsAssertions.ReadProblemDetailsAsync(response, "AZ-1126 item capturedAt offset-less");
+
+        // Assert
+        ProblemDetailsAssertions.AssertValidationProblem(problem, expectedStatus: 400, label: "AZ-1126 item capturedAt offset-less");
+        ProblemDetailsAssertions.AssertErrorsContainsMention(problem, expectedMention: "capturedAt", label: "AZ-1126 item capturedAt offset-less");
+
+        Console.WriteLine("  ✓ items[0].capturedAt without offset rejected");
+    }
+
     private static async Task ItemFlightIdMalformed_Returns400(string apiUrl, string secret)
     {
         Console.WriteLine();