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/featureFlags.ts

@@ -1,10 +1,21 @@
 import client from './client'
 
+/**
+ * Fetches all feature flags and their current states.
+ * @returns Promise resolving to a record of flag names to boolean values
+ * @throws {AxiosError} If the request fails
+ */
 export async function getFeatureFlags(): Promise<Record<string, boolean>> {
   const resp = await client.get<Record<string, boolean>>('/feature-flags')
   return resp.data
 }
 
+/**
+ * Updates one or more feature flags.
+ * @param payload - Record of flag names to new boolean values
+ * @returns Promise resolving to the update result
+ * @throws {AxiosError} If the update fails
+ */
 export async function updateFeatureFlags(payload: Record<string, boolean>) {
   const resp = await client.put('/feature-flags', payload)
   return resp.data