Fixes 4 high-severity findings from a comprehensive security audit:
1. Path traversal on ZIM file delete — resolve()+startsWith() containment
2. Path traversal on Map file delete — same pattern
3. Path traversal on docs read — same pattern (already used in rag_service)
4. SSRF on download endpoints — block private/internal IPs, require TLD
Also adds assertNotPrivateUrl() to content update endpoints.
Full audit report attached as admin/docs/security-audit-v1.md.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
