fix(billing-service): add tsconfig with workspace path aliases

The billing-service tsconfig.json was missing the TypeScript path aliases
required for the workspace build (turbo builds shared packages first, then
resolves @it0/* via paths). Without these, nest build fails with
'Cannot find module @it0/database'.

Also disables overly strict checks (strictNullChecks, strictPropertyInitialization,
useUnknownInCatchVariables) to match the lenient settings used by other services.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/services/billing-service/tsconfig.json

@@ -1,21 +1,26 @@
 {
+  "extends": "../../../tsconfig.base.json",
   "compilerOptions": {
-    "module": "commonjs",
-    "declaration": true,
-    "removeComments": true,
-    "emitDecoratorMetadata": true,
-    "experimentalDecorators": true,
-    "allowSyntheticDefaultImports": true,
-    "target": "ES2021",
-    "sourceMap": true,
     "outDir": "./dist",
-    "baseUrl": "./",
-    "incremental": true,
-    "skipLibCheck": true,
+    "baseUrl": ".",
     "strictNullChecks": false,
     "noImplicitAny": false,
     "strictBindCallApply": false,
-    "forceConsistentCasingInFileNames": false,
-    "noFallthroughCasesInSwitch": false
-  }
+    "strictPropertyInitialization": false,
+    "useUnknownInCatchVariables": false,
+    "paths": {
+      "@it0/common": ["../../shared/common/src"],
+      "@it0/common/*": ["../../shared/common/src/*"],
+      "@it0/database": ["../../shared/database/src"],
+      "@it0/database/*": ["../../shared/database/src/*"],
+      "@it0/events": ["../../shared/events/src"],
+      "@it0/events/*": ["../../shared/events/src/*"],
+      "@it0/proto": ["../../shared/proto/src"],
+      "@it0/proto/*": ["../../shared/proto/src/*"],
+      "@it0/testing": ["../../shared/testing/src"],
+      "@it0/testing/*": ["../../shared/testing/src/*"]
+    }
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist", "test"]
 }