scripts: Fix gate-4-startup.ps1 for DEVELOPMENT environment and DB credentials

Fixes:
- Add ASPNETCORE_ENVIRONMENT=Development to load appsettings.Development.json
  (enables DevelopmentHeaderAuthenticationHandler for header-based auth)
- Correct KARTSELL_POSTGRES connection string to match appsettings.json:
  * Database: kartsell → kartselldb
  * Password: kartsell → kartsell4321@!
  * Host: localhost → 127.0.0.1
- Correct API key environment variable names:
  * KRX_OPENAPI → KRX_API_KEY
  * Add OPENDART_API for completeness

Impact:
- Host now starts in Development mode with proper authentication handler
- Shadow Run API test passes (HTTP 202 Accepted)
- Gate 3 validation successful: Job 893 queued with 253-day window

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

scripts/gate-4-startup.ps1

@@ -49,9 +49,11 @@ if (-not $SkipDbUp) {
 Write-Host "[3/3] Starting Host (DEBUG mode)..." -ForegroundColor Yellow
 Write-Host ""
 
-# Set required environment variables
-$env:KARTSELL_POSTGRES = "Host=localhost;Port=5432;Database=kartsell;Username=kartsell;Password=kartsell"
-$env:KRX_OPENAPI = "stub-key-for-testing"
+# Set required environment variables (must match appsettings.json)
+$env:ASPNETCORE_ENVIRONMENT = "Development"  # Load appsettings.Development.json (DevelopmentHeaderAuthenticationHandler)
+$env:KARTSELL_POSTGRES = "Host=127.0.0.1;Port=5432;Database=kartselldb;Username=kartsell;Password=kartsell4321@!"
+$env:KRX_API_KEY = "stub-key-for-testing"
+$env:OPENDART_API = "stub-key-for-testing"
 
 # Display startup info
 Write-Host "Configuration: $Environment" -ForegroundColor Cyan