feat: infinite scroll, HEIC support, likes fix, security & backup

- Fix liked hearts not showing after page reload (sync useEffect in CatCard)
- Replace pagination with infinite scroll (useInfiniteQuery + IntersectionObserver)
- Add HEIC/HEIF/AVIF image support with sharp conversion to JPEG on upload
- Validate MIME type alongside file extension to prevent spoofing
- Limit caption to 500 chars; increase upload size limit to 20MB
- Delete physical file from uploads/ when post is removed (cats + admin routes)
- Add WAL journal mode and graceful shutdown (SIGTERM/SIGINT) to db
- Add rate limiter for upload endpoint (10 req / 15 min)
- Add pre-deploy backup of data.db and uploads/ to deploy.sh (keeps last 10)
- Add plan_modernization.md with ideas for future improvements

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

server/package.json

@@ -15,6 +15,7 @@
     "helmet": "^8.2.0",
     "jsonwebtoken": "^9.0.2",
     "multer": "^1.4.5-lts.1",
+    "sharp": "^0.33.0",
     "sql.js": "^1.14.1"
   },
   "devDependencies": {