fix(devvit): override protobufjs to ^7.6.5

@devvit/protos pins protobufjs exactly (7.5.8 here, 7.6.3 in the latest
release), so Dependabot security updates cannot resolve the open
advisories. Force a patched 7.x through npm overrides.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: bakerboy448

devvit/package-lock.json

@@ -22,5 +22,8 @@
     "devvit": "1.0.0",
     "typescript": "5.8.3",
     "vitest": "4.1.11"
+  },
+  "overrides": {
+    "protobufjs": "^7.6.5"
   }
 }