docs: JWT authentication implementation guide and production config

- Added comprehensive JWT_AUTHENTICATION.md documentation
- Covers backend (JwtAuthenticationHandler, LoginEndpoint) and frontend (useAuthApi, LoginPage)
- Includes configuration for development and production modes
- Security considerations and best practices
- Token refresh enhancement recommendations
- Testing guide and troubleshooting
- API contract documentation
- Deployment checklist

- Added appsettings.Release.json for production JWT configuration
- Placeholders for environment-specific values (JWT_KEY)
- Proper listen address (0.0.0.0:5002) for containerized deployments

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

src/KArtSell.Host/appsettings.Release.json

@@ -0,0 +1,26 @@
+{
+  "Kestrel": {
+    "Endpoints": {
+      "Http": {
+        "Url": "http://0.0.0.0:5002"
+      }
+    }
+  },
+  "Authentication": {
+    "Mode": "JWT"
+  },
+  "Jwt": {
+    "Key": "${JWT_KEY}",
+    "Issuer": "KArtSell.Aegis",
+    "Audience": "KArtSell.Aegis",
+    "ExpirationMinutes": 60
+  },
+  "Serilog": {
+    "MinimumLevel": {
+      "Default": "Information",
+      "Override": {
+        "Microsoft.AspNetCore": "Warning"
+      }
+    }
+  }
+}