chore: Protect credentials by removing .env files from tracking

- Remove .env and .env.local from git tracking (contain GitHub OAuth secrets)
- Add backend .gitignore to prevent future accidental commits of credentials
- Keep .env.example as template for developers

Security: GitHub OAuth credentials and other secrets are now protected from public repo.

🤖 Generated with Claude Code

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: Spacehunterz

apps/dashboard/backend/.env

@@ -0,0 +1,22 @@
+# Environment variables - NEVER commit credentials
+.env
+.env.local
+.env.*.local
+
+# Python
+__pycache__/
+*.pyc
+*.pyo
+*.egg-info/
+.pytest_cache/
+.coverage
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+
+# Logs
+*.log
+backend.log