feat(knowledge): add markdown rendering (KNOW-004)

- Install marked, marked-highlight, marked-gfm-heading-id, sanitize-html
- Create markdown utility with GFM support (tables, task lists, strikethrough)
- Add code syntax highlighting with highlight.js
- Implement XSS sanitization for security
- Update knowledge service to use markdown renderer
- Add comprehensive test suite (34 tests, all passing)
- Generate IDs for headers for deep linking
- Cache rendered HTML in database for performance

apps/api/package.json

@@ -36,9 +36,13 @@
     "better-auth": "^1.4.17",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.14.3",
+    "highlight.js": "^11.11.1",
     "marked": "^17.0.1",
+    "marked-gfm-heading-id": "^4.1.3",
+    "marked-highlight": "^2.2.3",
     "reflect-metadata": "^0.2.2",
     "rxjs": "^7.8.1",
+    "sanitize-html": "^2.17.0",
     "slugify": "^1.6.6"
   },
   "devDependencies": {
@@ -49,7 +53,9 @@
     "@nestjs/testing": "^11.1.12",
     "@swc/core": "^1.10.18",
     "@types/express": "^5.0.1",
+    "@types/highlight.js": "^10.1.0",
     "@types/node": "^22.13.4",
+    "@types/sanitize-html": "^2.16.0",
     "@vitest/coverage-v8": "^4.0.18",
     "express": "^5.2.1",
     "prisma": "^6.19.2",