feat: validate backend runtime configuration

backend/libs/configuration/src/configuration.module.ts

@@ -0,0 +1,9 @@
+import { Global, Module } from '@nestjs/common';
+import { appEnvironmentProvider } from './environment';
+
+@Global()
+@Module({
+  providers: [appEnvironmentProvider],
+  exports: [appEnvironmentProvider],
+})
+export class ConfigurationModule {}

backend/libs/configuration/src/environment.spec.ts

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+import { loadEnvironment } from './environment';
+
+describe('loadEnvironment', () => {
+  it('requires database and redis URLs', () => {
+    expect(() => loadEnvironment({})).toThrow('DATABASE_URL');
+  });
+
+  it('returns build metadata without secrets', () => {
+    expect(
+      loadEnvironment({
+        DATABASE_URL: 'postgresql://u:p@postgres:5432/db',
+        REDIS_URL: 'redis://redis:6379',
+        APP_VERSION: '1.2.3',
+        TEAMCITY_BUILD_NUMBER: '42',
+        SOURCE_REVISION: 'abc123',
+      }),
+    ).toEqual({
+      databaseUrl: 'postgresql://u:p@postgres:5432/db',
+      redisUrl: 'redis://redis:6379',
+      appVersion: '1.2.3',
+      teamCityBuildNumber: '42',
+      sourceRevision: 'abc123',
+    });
+  });
+});

backend/libs/configuration/src/environment.ts

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+export interface AppEnvironment {
+  databaseUrl: string;
+  redisUrl: string;
+  appVersion: string;
+  teamCityBuildNumber: string;
+  sourceRevision: string;
+}
+
+function required(env: NodeJS.ProcessEnv, name: string): string {
+  const value = env[name]?.trim();
+  if (!value) throw new Error(`Missing required environment variable: ${name}`);
+  return value;
+}
+
+export function loadEnvironment(env: NodeJS.ProcessEnv): AppEnvironment {
+  return {
+    databaseUrl: required(env, 'DATABASE_URL'),
+    redisUrl: required(env, 'REDIS_URL'),
+    appVersion: env.APP_VERSION?.trim() || 'dev',
+    teamCityBuildNumber: env.TEAMCITY_BUILD_NUMBER?.trim() || 'local',
+    sourceRevision: env.SOURCE_REVISION?.trim() || 'local',
+  };
+}
+
+export const APP_ENVIRONMENT = Symbol('APP_ENVIRONMENT');
+
+export const appEnvironmentProvider = {
+  provide: APP_ENVIRONMENT,
+  useFactory: () => loadEnvironment(process.env),
+};

backend/libs/configuration/src/index.ts

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+export * from './environment';
+export * from './configuration.module';