Platform stats publish writes a new versioned file each run, so its immutable header holds again

The versioned stats file is served public, max-age=31536000, immutable, and
the manifest platform-stats.json names the current one. Its per-day name
(19e9713a) was unique while maintenance ran once a night; since 82919d47 it
runs every 2 hours and rewrote that file up to 12 times a day, so caches kept
the first copy of the day. The name now carries the publish instant to the
second in invariant culture (platform-stats.2026-10-03T060004Z.json).

Headers and access tier travel with the upload instead of two follow-up
calls, which left the blob briefly without Cache-Control and content type,
and kept it that way (with the manifest still on the old file) when one of
them failed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N4ddfosgdeCwXo7NvB9xd2

src/Backend/AutopilotMonitor.Functions.Tests/PlatformStatsPublishTests.cs

@@ -0,0 +1,45 @@
+using System.Globalization;
+using AutopilotMonitor.Functions.Services;
+
+namespace AutopilotMonitor.Functions.Tests;
+
+/// <summary>
+/// The versioned platform-stats file is served immutable for a year, so every publish must write
+/// a new name. Maintenance publishes every 2 hours; the per-day name it used to write was rewritten
+/// up to 12 times a day, and caches kept the first copy of the day.
+/// </summary>
+public class PlatformStatsPublishTests
+{
+    private static readonly DateTime Publish = new(2026, 10, 3, 6, 0, 4, DateTimeKind.Utc);
+
+    [Fact]
+    public void Versioned_file_name_carries_the_publish_instant_to_the_second()
+    {
+        Assert.Equal("platform-stats.2026-10-03T060004Z.json", MaintenanceService.PlatformStatsVersionedFileName(Publish));
+    }
+
+    [Fact]
+    public void Two_publishes_of_one_day_get_different_names()
+    {
+        var early = MaintenanceService.PlatformStatsVersionedFileName(new DateTime(2026, 10, 3, 2, 0, 3, DateTimeKind.Utc));
+        var later = MaintenanceService.PlatformStatsVersionedFileName(new DateTime(2026, 10, 3, 4, 0, 3, DateTimeKind.Utc));
+
+        Assert.NotEqual(early, later);
+    }
+
+    [Fact]
+    public void Versioned_file_name_ignores_the_current_culture()
+    {
+        var previous = CultureInfo.CurrentCulture;
+        try
+        {
+            // th-TH counts years in the Buddhist era (2569 for 2026).
+            CultureInfo.CurrentCulture = new CultureInfo("th-TH");
+            Assert.Equal("platform-stats.2026-10-03T060004Z.json", MaintenanceService.PlatformStatsVersionedFileName(Publish));
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = previous;
+        }
+    }
+}

src/Backend/AutopilotMonitor.Functions/Services/MaintenanceService.Aggregation.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Globalization;
 using System.IO;
 using System.Linq;
 using System.Text;
@@ -795,7 +796,7 @@ namespace AutopilotMonitor.Functions.Services
 
                 var containerClient = new BlobContainerClient(new Uri(containerSasUrl));
                 var generatedAtUtc = DateTime.UtcNow;
-                var versionedFileName = $"platform-stats.{generatedAtUtc:yyyy-MM-dd}.json";
+                var versionedFileName = PlatformStatsVersionedFileName(generatedAtUtc);
 
                 var versionedPayload = new
                 {
@@ -831,19 +832,34 @@ namespace AutopilotMonitor.Functions.Services
             }
         }
 
+        /// <summary>
+        /// Name of the versioned stats file of one publish. The file is served immutable for a year
+        /// (<see cref="PlatformStatsVersionedCacheControl"/>), so a name must never be written twice:
+        /// maintenance publishes every 2 hours, and the per-day name of the once-a-night schedule was
+        /// rewritten up to 12 times a day under that header — caches kept the first copy of the day.
+        /// Browsers revalidate only the manifest, which names the current file.
+        /// </summary>
+        internal static string PlatformStatsVersionedFileName(DateTime generatedAtUtc) =>
+            $"platform-stats.{generatedAtUtc.ToString("yyyy-MM-dd'T'HHmmss'Z'", CultureInfo.InvariantCulture)}.json";
+
         private async Task UploadJsonBlobAsync(BlobContainerClient containerClient, string blobName, object payload, string cacheControl)
         {
             var blobClient = containerClient.GetBlobClient(blobName);
             var json = JsonConvert.SerializeObject(payload);
             await using var stream = new MemoryStream(Encoding.UTF8.GetBytes(json));
 
-            await blobClient.UploadAsync(stream, overwrite: true);
-            await blobClient.SetHttpHeadersAsync(new BlobHttpHeaders
+            // Headers and tier travel with the upload, which still overwrites (the manifest relies on
+            // it). Set in follow-up calls, the blob was briefly served without Cache-Control and
+            // content type, and stayed that way when a follow-up call failed.
+            await blobClient.UploadAsync(stream, new BlobUploadOptions
             {
-                ContentType = "application/json; charset=utf-8",
-                CacheControl = cacheControl
+                HttpHeaders = new BlobHttpHeaders
+                {
+                    ContentType = "application/json; charset=utf-8",
+                    CacheControl = cacheControl
+                },
+                AccessTier = AccessTier.Hot
             });
-            await blobClient.SetAccessTierAsync(AccessTier.Hot);
         }
 
         /// <summary>

src/Backend/AutopilotMonitor.Functions/Services/MaintenanceService.cs

@@ -60,6 +60,7 @@ namespace AutopilotMonitor.Functions.Services
 
         private const string PlatformStatsAliasFileName = "platform-stats.json";
         private const string PlatformStatsAliasCacheControl = "public, max-age=300, stale-while-revalidate=86400";
+        // Holds only because every publish writes a new name (PlatformStatsVersionedFileName).
         private const string PlatformStatsVersionedCacheControl = "public, max-age=31536000, immutable";
 
         public MaintenanceService(