fix(deps): High-Advisories schliessen

pnpm audit --prod meldete 5 high:
- next 16.3.6 -> 16.3.8, eslint-config-next mitgezogen
- sharp 0.35.5 direkt und per Override (next pinnt sonst 0.35.4)
- source-map-js >= 1.2.2 (transitiv ueber next)
- mysql2 >= 3.23.1 und deepmerge-ts >= 8.0.0 (prisma-CLI, gleiche
  Untergrenzen wie im Wabenhain-Repo)

Lokal geprueft: lint, typecheck, 132 Tests, next build gruen; audit sauber.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

package.json

@@ -27,14 +27,14 @@
     "@prisma/client": "7.10.0",
     "better-auth": "1.7.6",
     "motion": "13.4.4",
-    "next": "16.3.6",
+    "next": "16.3.8",
     "nodemailer": "10.0.10",
     "pg": "8.23.0",
     "qrcode": "1.5.4",
     "react": "19.3.0",
     "react-dom": "19.3.0",
     "server-only": "0.0.1",
-    "sharp": "0.35.4",
+    "sharp": "0.35.5",
     "stripe": "22.6.2",
     "zod": "4.6.5"
   },
@@ -51,7 +51,7 @@
     "dotenv": "18.0.4",
     "embedded-postgres": "18.4.0-beta.17",
     "eslint": "^9.39.5",
-    "eslint-config-next": "16.3.6",
+    "eslint-config-next": "16.3.8",
     "ffmpeg-static": "5.3.0",
     "prisma": "7.10.0",
     "tailwindcss": "4.3.3",