revert: roll back .NET 10 upgrade — Flex stamp missing runtime

Reverts:
- 8dacce5c "chore(backend): bump optional NuGet patches + fix SYSLIB0057"
- 2f68fe69 "chore(backend): upgrade Functions from .NET 8 to .NET 10 isolated"

After deploy succeeded and the Flex Consumption stack was switched to
.NET 10 Isolated in the portal, the dotnet-isolated worker died on every
boot with: dotnet exited with code 150 (FrameworkMissingFailure) and
"Exceeded language worker restart retry count for runtime:dotnet-isolated".
The deployed package was correct (TFM net10.0, FrameworkReference 10.0.0,
all package bumps applied) and the runtime stack property was set to
dotnet-isolated 10.0, but the .NET 10 runtime is not present on the
Flex Consumption stamp in West Europe even though Microsoft lists
DOTNET-ISOLATED|10.0 as supported. Restart and stop+start did not help.

Recovery sequence:
1. Stack switched back to .NET 8 Isolated (manual portal action).
2. Re-deployed last known-good net8.0 commit 637bd9a1 via temp branch
   rollback-net8 (now safe to delete).
3. /api/health back to 200.

Follow-ups:
- Re-apply security fixes (DataProtection 10.0.7 pin GHSA-9mv3-2cwr-p262,
  SYSLIB0057 X509CertificateLoader migration) on net8.0 in a separate PR.
- Open Microsoft support: ".NET 10 Isolated worker on Flex Consumption
  WestEurope fails with FrameworkMissingFailure despite stack listed as
  supported."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/deploy-backend.yml

@@ -16,11 +16,10 @@ jobs:
         with:
           creds: ${{ secrets.AZURE_CREDENTIALS }}
 
-      - name: Setup .NET 10
+      - name: Setup .NET 8
         uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: '10.0.x'
-          dotnet-quality: 'ga'
+          dotnet-version: '8.0.x'
 
       - name: Restore dependencies
         run: dotnet restore src/Backend/AutopilotMonitor.Functions/AutopilotMonitor.Functions.csproj

global.json

@@ -1,7 +0,0 @@
-{
-  "sdk": {
-    "version": "10.0.100",
-    "rollForward": "latestFeature",
-    "allowPrerelease": false
-  }
-}

src/Backend/AutopilotMonitor.Functions.Tests/AutopilotMonitor.Functions.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net10.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>

src/Backend/AutopilotMonitor.Functions/AutopilotMonitor.Functions.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net10.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <AzureFunctionsVersion>V4</AzureFunctionsVersion>
     <OutputType>Exe</OutputType>
     <ImplicitUsings>enable</ImplicitUsings>
@@ -28,25 +28,23 @@
 
   <ItemGroup>
     <FrameworkReference Include="Microsoft.AspNetCore.App" />
-    <PackageReference Include="Azure.Identity" Version="1.21.0" />
+    <PackageReference Include="Azure.Identity" Version="1.19.0" />
     <PackageReference Include="Microsoft.ApplicationInsights.WorkerService" Version="2.23.0" />
     <PackageReference Include="Microsoft.Azure.Functions.Worker" Version="2.51.0" />
     <PackageReference Include="Microsoft.Azure.Functions.Worker.ApplicationInsights" Version="2.50.0" />
     <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore" Version="2.1.0" />
     <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.SignalRService" Version="2.0.1" />
-    <PackageReference Include="Microsoft.Azure.SignalR.Management" Version="1.33.0" />
+    <PackageReference Include="Microsoft.Azure.SignalR.Management" Version="1.29.0" />
     <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.Timer" Version="4.3.1" />
     <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.Storage.Queues" Version="5.5.4" />
     <PackageReference Include="Microsoft.Azure.Functions.Worker.Sdk" Version="2.0.7" />
-    <PackageReference Include="Azure.Data.Tables" Version="12.11.0" />
-    <PackageReference Include="Azure.Storage.Blobs" Version="12.27.0" />
-    <PackageReference Include="Azure.Storage.Queues" Version="12.25.0" />
-    <PackageReference Include="Newtonsoft.Json" Version="13.0.4" />
-    <PackageReference Include="Microsoft.Identity.Web" Version="4.8.0" />
-    <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="10.0.7" />
-    <!-- Pin to override transitive 10.0.0 brought in by Microsoft.Identity.Web; 10.0.0 has GHSA-9mv3-2cwr-p262 (critical). -->
-    <PackageReference Include="Microsoft.AspNetCore.DataProtection" Version="10.0.7" />
-    <PackageReference Include="Microsoft.Extensions.Http.Polly" Version="10.0.7" />
+    <PackageReference Include="Azure.Data.Tables" Version="12.9.1" />
+    <PackageReference Include="Azure.Storage.Blobs" Version="12.22.2" />
+    <PackageReference Include="Azure.Storage.Queues" Version="12.23.0" />
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
+    <PackageReference Include="Microsoft.Identity.Web" Version="4.3.0" />
+    <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="8.0.11" />
+    <PackageReference Include="Microsoft.Extensions.Http.Polly" Version="8.0.25" />
     <PackageReference Include="Resend" Version="0.2.1" />
   </ItemGroup>
 

src/Backend/AutopilotMonitor.Functions/Security/CertificateValidator.cs

@@ -66,7 +66,7 @@ namespace AutopilotMonitor.Functions.Security
 
                 // Parse certificate from Base64
                 var certBytes = Convert.FromBase64String(certificateBase64);
-                var certificate = X509CertificateLoader.LoadCertificate(certBytes);
+                var certificate = new X509Certificate2(certBytes);
                 var thumbprint = certificate.Thumbprint;
 
                 logger?.LogDebug($"Validating certificate: Subject={certificate.Subject}, Issuer={certificate.Issuer}, Thumbprint={thumbprint}");