feat: add CODEOWNERS for automated review routing

Route PRs to appropriate reviewers based on file paths:
- Default: visitran-reviewers-l1 (Tahier, Salahudeen, Deepak G)
- Backend + Core: also visitran-reviewers-l2 (Jaseem, Ritwik, Abhishek, Hari)
- CI/CD + Docker: visitran-admins

Branch protection "Require CODEOWNERS review" is already enabled.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abhizipstack

.github/CODEOWNERS

@@ -0,0 +1,22 @@
+# Default — all PRs require review from L1 reviewers
+* @Zipstack/visitran-reviewers-l1
+
+# Backend
+/backend/ @Zipstack/visitran-reviewers-l1 @Zipstack/visitran-reviewers-l2
+
+# Frontend
+/frontend/ @Zipstack/visitran-reviewers-l1
+
+# Core library
+/visitran/ @Zipstack/visitran-reviewers-l2
+
+# Tests
+/tests/ @Zipstack/visitran-reviewers-l1
+
+# CI/CD and infrastructure
+/.github/ @Zipstack/visitran-admins
+/docker/ @Zipstack/visitran-admins
+
+# Documentation
+/*.md @Zipstack/visitran-reviewers-l1
+/docs/ @Zipstack/visitran-reviewers-l1