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/settings.ts

@@ -1,14 +1,28 @@
 import client from './client'
 
+/** Map of setting keys to string values. */
 export interface SettingsMap {
   [key: string]: string
 }
 
+/**
+ * Fetches all application settings.
+ * @returns Promise resolving to SettingsMap
+ * @throws {AxiosError} If the request fails
+ */
 export const getSettings = async (): Promise<SettingsMap> => {
   const response = await client.get('/settings')
   return response.data
 }
 
+/**
+ * Updates a single application setting.
+ * @param key - The setting key to update
+ * @param value - The new value for the setting
+ * @param category - Optional category for organization
+ * @param type - Optional type hint for the setting
+ * @throws {AxiosError} If the update fails
+ */
 export const updateSetting = async (key: string, value: string, category?: string, type?: string): Promise<void> => {
   await client.post('/settings', { key, value, category, type })
 }