Rollback to SQLite-only version and fix all broken tests

- Rolled back to commit 3c850e4 (last working SQLite version)
- Removed all PocketBase and ChromaDB code and files
- Fixed broken tests in security_retrieval_service (pointer dereferencing)
- Fixed database schema: moved security tables to main schema (not FTS5-only)
- Added missing external_id column to attack_tactics table
- Fixed time parsing in GetSecurityDataSummary (SQLite returns strings)
- Fixed column name mismatches (last_update -> last_updated)
- Cleaned up outdated documentation files
- All tests now passing
- Build verified and working

Author: rainmana