chore: implement instruction compliance remediation

- Replace Go interface{} with any (Go 1.18+ standard)
- Add database indexes to frequently queried model fields
- Add JSDoc documentation to frontend API client methods
- Remove deprecated docker-compose version keys
- Add concurrency groups to all 25 GitHub Actions workflows
- Add YAML front matter and fix H1→H2 headings in docs

Coverage: Backend 85.5%, Frontend 87.73%
Security: No vulnerabilities detected

Refs: docs/plans/instruction_compliance_spec.md

.github/workflows/docs.yml

@@ -21,6 +21,9 @@ concurrency:
   group: "pages"
   cancel-in-progress: false
 
+env:
+  NODE_VERSION: '24.12.0'
+
 jobs:
   build:
     name: Build Documentation
@@ -35,7 +38,7 @@ jobs:
       - name: 🔧 Set up Node.js
         uses: actions/setup-node@395ad3262231945c25e8478fd5baf05154b1d79f # v6
         with:
-          node-version: '24.12.0'
+          node-version: ${{ env.NODE_VERSION }}
 
       # Step 3: Create a beautiful docs site structure
       - name: 📝 Build documentation site