FRE-4510: Implement feature flag checks for spam classification

- Add runtime flag evaluation from FLAG_<KEY> environment variables
- Add enableCallAnalysis flag check to analyzeCall() and interceptCall()
- Add enableFeedbackLoop flag check to recordFeedback()
- Add 19 tests for feature flag behavior (checkFeatureFlag, getters, service integration)
- Add vitest config and test script to spamshield package

services/spamshield/package.json

@@ -7,6 +7,7 @@
     "build": "tsc",
     "dev": "tsx watch src/index.ts",
     "lint": "eslint src/",
+    "test": "vitest run",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {