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

frontend/src/api/client.ts

@@ -1,11 +1,19 @@
 import axios from 'axios';
 
+/**
+ * Pre-configured Axios instance for API communication.
+ * Includes base URL, credentials, and timeout settings.
+ */
 const client = axios.create({
   baseURL: '/api/v1',
   withCredentials: true, // Required for HttpOnly cookie transmission
   timeout: 30000, // 30 second timeout
 });
 
+/**
+ * Sets or clears the Authorization header for API requests.
+ * @param token - JWT token to set, or null to clear authentication
+ */
 export const setAuthToken = (token: string | null) => {
   if (token) {
     client.defaults.headers.common.Authorization = `Bearer ${token}`;