What does this PR do?

The RetainDB memory plugin reads RETAINDB_BASE_URL from an environment
variable and uses it directly in HTTP requests without any validation.

If RETAINDB_BASE_URL is set to an internal address (e.g.
http://169.254.169.254 on AWS, http://metadata.google.internal on GCP,
or any private network service), every memory read and write would silently
make requests to that internal address — a classic SSRF attack vector.

Fix

Added an is_safe_url() check in initialize() after reading
RETAINDB_BASE_URL. If the URL resolves to a private/internal address,
it is reset to the safe default endpoint with a warning log.

This is consistent with the SSRF protection already applied in:

• tools/homeassistant_tool.py (HASS_URL)
• tools/web_tools.py
• tools/vision_tools.py
• gateway/platforms/wecom.py

Type of Change

• 🔒 Security fix (SSRF)

Checklist

• Read the Contributing Guide
• Commit messages follow Conventional Commits
• Consistent with existing SSRF protection pattern in Hermes
• No behavior change for legitimate RetainDB endpoints
• ImportError handled gracefully