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

docs/database-maintenance.md

@@ -1,4 +1,9 @@
-# Database Maintenance
+---
+title: Database Maintenance
+description: SQLite database maintenance guide for Charon. Covers backups, recovery, and troubleshooting database issues.
+---
+
+## Database Maintenance
 
 Charon uses SQLite as its embedded database. This guide explains how the database
 is configured, how to maintain it, and what to do if something goes wrong.