feat(browser): add environment, credentials, and script logger to POST /exec

- ExecDto gains optional `environment` and `credentials` fields
- BrowserService.exec forwards both to CodeExecutorService.execute so
  helpers.env, helpers.getEnvUrl(), and helpers.getCredential() work
  identically to scenario-scheduler steps
- script console.log/warn/error now routed through TraceLogger with
  session label prefix
- integration tests cover env/creds injection and missing-alias error

server/src/browser/dto/exec.dto.ts

@@ -1,5 +1,6 @@
 import { ApiProperty, ApiPropertyOptional } from "@nestjs/swagger";
-import { IsOptional, IsString, IsUrl } from "class-validator";
+import { IsObject, IsOptional, IsString, IsUrl } from "class-validator";
+import type { EnvironmentData } from "../../environment/environment.entity";
 
 export class ExecDto {
   @ApiPropertyOptional({
@@ -27,4 +28,22 @@ export class ExecDto {
   })
   @IsString()
   code: string;
+
+  @ApiPropertyOptional({
+    description:
+      "Key/value map of environment variables available via `helpers.env` and `helpers.getEnvUrl()` in the script.",
+    example: { BASE_URL: "https://example.com" },
+  })
+  @IsOptional()
+  @IsObject()
+  environment?: EnvironmentData;
+
+  @ApiPropertyOptional({
+    description:
+      "Key/value map of credentials available via `helpers.getCredential()` in the script.",
+    example: { admin: { username: "user", password: "pass" } },
+  })
+  @IsOptional()
+  @IsObject()
+  credentials?: Record<string, unknown>;
 }