Harden auth flows and add backend quality gate

scripts/run_backend_quality_gate.ps1

@@ -0,0 +1,59 @@
+$ErrorActionPreference = "Stop"
+
+$repoRoot = Resolve-Path "$PSScriptRoot\.."
+Set-Location $repoRoot
+
+function Assert-LastExitCode {
+    param([Parameter(Mandatory = $true)][string]$CommandName)
+
+    if ($LASTEXITCODE -ne 0) {
+        throw "$CommandName failed with exit code $LASTEXITCODE."
+    }
+}
+
+function Get-PythonCommand {
+    $venvPython = Join-Path $repoRoot ".venv\Scripts\python.exe"
+    if (Test-Path $venvPython) {
+        return $venvPython
+    }
+    return "python"
+}
+
+function Ensure-PythonModule {
+    param(
+        [Parameter(Mandatory = $true)][string]$PythonExe,
+        [Parameter(Mandatory = $true)][string]$ModuleName,
+        [Parameter(Mandatory = $true)][string]$PackageName
+    )
+
+    & $PythonExe -c "import importlib.util, sys; sys.exit(0 if importlib.util.find_spec('$ModuleName') else 1)"
+    if ($LASTEXITCODE -eq 0) {
+        return
+    }
+
+    Write-Host "Installing missing Python package: $PackageName"
+    & $PythonExe -m pip install $PackageName
+    Assert-LastExitCode -CommandName "python -m pip install $PackageName"
+}
+
+$pythonExe = Get-PythonCommand
+
+Write-Host "Installing backend Python requirements"
+& $pythonExe -m pip install -r (Join-Path $repoRoot "backend\requirements.txt")
+Assert-LastExitCode -CommandName "python -m pip install -r backend/requirements.txt"
+
+Write-Host "Running Python dependency integrity check"
+& $pythonExe -m pip check
+Assert-LastExitCode -CommandName "python -m pip check"
+
+Ensure-PythonModule -PythonExe $pythonExe -ModuleName "pip_audit" -PackageName "pip-audit"
+
+Write-Host "Running Python vulnerability scan"
+& $pythonExe -m pip_audit -r (Join-Path $repoRoot "backend\requirements.txt") --progress-spinner off --desc
+Assert-LastExitCode -CommandName "python -m pip_audit"
+
+Write-Host "Running backend unit tests"
+& $pythonExe -m unittest discover -s backend/tests -p "test_*.py" -v
+Assert-LastExitCode -CommandName "python -m unittest discover"
+
+Write-Host "Backend quality gate passed"