fix(backend): Fix foreign key constraint error in tenant registration

Root Cause:
- Schema mismatch between user_tenant_roles table (identity schema) and
  users/tenants tables (default/public schema)
- PostgreSQL FK constraints couldn't find referenced tables due to
  schema mismatch
- Error: "violates foreign key constraint FK_user_tenant_roles_tenants_tenant_id"

Solution:
1. Moved users and tenants tables to identity schema
2. Created migration MoveTablesToIdentitySchemaAndAddIndexes
3. All Identity module tables now in consistent identity schema
4. Added performance index for users.email lookups

Changes:
- Updated TenantConfiguration.cs to use identity schema
- Updated UserConfiguration.cs to use identity schema
- Created migration to move tables to identity schema
- Removed old AddPerformanceIndexes migration (referenced wrong schema)
- Created new AddPerformanceIndexes migration
- Added test script test-tenant-registration.ps1

Test Results:
- Tenant registration now works successfully
- User, Tenant, and UserTenantRole all insert correctly
- FK constraints validate properly
- Access token and refresh token generated successfully

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

colaflow-api/test-tenant-registration.ps1

@@ -0,0 +1,35 @@
+# Test Tenant Registration API
+
+$body = @{
+    tenantName = "Test Corp"
+    tenantSlug = "test-corp-$(Get-Random)"
+    subscriptionPlan = "Professional"
+    adminEmail = "admin@testcorp.com"
+    adminPassword = "Admin@1234"
+    adminFullName = "Test Admin"
+} | ConvertTo-Json
+
+Write-Host "Testing Tenant Registration API..." -ForegroundColor Cyan
+Write-Host "Request Body:" -ForegroundColor Yellow
+Write-Host $body
+
+try {
+    $response = Invoke-RestMethod -Uri "http://localhost:5167/api/tenants/register" `
+        -Method Post `
+        -ContentType "application/json" `
+        -Body $body
+
+    Write-Host ""
+    Write-Host "SUCCESS! Registration completed successfully!" -ForegroundColor Green
+    Write-Host ""
+    Write-Host "Response:" -ForegroundColor Yellow
+    Write-Host ($response | ConvertTo-Json -Depth 10)
+    Write-Host ""
+    Write-Host "Access Token:" -ForegroundColor Cyan
+    Write-Host $response.accessToken
+} catch {
+    Write-Host ""
+    Write-Host "FAILED! Registration failed!" -ForegroundColor Red
+    Write-Host "Error: $($_.Exception.Message)" -ForegroundColor Red
+    Write-Host "Details: $($_.ErrorDetails.Message)" -ForegroundColor Red
+}