feat: Checkfirst connector - Pravda network infrastructure & improvements

[herve-checkfirst]

Summary

• Add one-off Pravda network infrastructure bundle (36 pravda-XX.com domains, 60+ news-pravda.com subdomains, shared hosting IP 178.21.15.85) attributed to the Portal-Kombat intrusion set per SGDSN/VIGINUM reports
• Improve STIX object model: add Infrastructure, Campaign, IntrusionSet SDOs with deterministic IDs and proper temporal relationships
• Use schedule_iso() for standard external-import scheduling
• Update README with full STIX object model documentation and infrastructure bundle details

Changes

• pravda_network.py — new module with Pravda domain/IP data and subdomain mappings
• main_logic.py — infrastructure bundle on first run, improved article-to-STIX mapping with Infrastructure objects
• converter_to_stix.py — new STIX object builders (Infrastructure, IPv4, Campaign, IntrusionSet, relationships)
• connector.py — switch to schedule_iso() scheduling
• settings.py / types.py — minor additions
• README.md — updated with infrastructure bundle docs and STIX model diagrams
• .circleci/config.yml — CI fix for LTS release builds

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