chore: remove Dependabot config

Dependency bumps will be handled manually to avoid noisy PRs.

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

.github/dependabot.yml

@@ -1,10 +0,0 @@
-version: 2
-updates:
-  - package-ecosystem: "npm"
-    directory: "/frontend"
-    schedule:
-      interval: "weekly"
-  - package-ecosystem: "pip"
-    directory: "/backend"
-    schedule:
-      interval: "weekly"