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/codecov-upload.yml

@@ -7,6 +7,14 @@ on:
       - development
       - 'feature/**'
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  GO_VERSION: '1.25.5'
+  NODE_VERSION: '24.12.0'
+
 permissions:
   contents: read
 
@@ -23,7 +31,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@4dc6199c7b1a012772edbd06daecab0f50c9053c # v6
         with:
-          go-version: '1.25.5'
+          go-version: ${{ env.GO_VERSION }}
           cache-dependency-path: backend/go.sum
 
       - name: Run Go tests with coverage
@@ -54,7 +62,7 @@ jobs:
       - name: Set up Node.js
         uses: actions/setup-node@395ad3262231945c25e8478fd5baf05154b1d79f # v6
         with:
-          node-version: '24.12.0'
+          node-version: ${{ env.NODE_VERSION }}
           cache: 'npm'
           cache-dependency-path: frontend/package-lock.json