fix(extract): initialize detection for pathless Postgres

[oleksii-tumanov]

Summary

• initialize an empty detection result for PostgreSQL-only extraction without a source path
• skip semantic-cache pruning when there is no filesystem corpus
• preserve protected backups, then invalidate the stale file manifest before writing the DB-only graph
• preserve the existing filesystem detection paths unchanged
• cover both CLI forms and both output paths through a file → DB → file rebuild

Result

Pathless PostgreSQL extraction reaches the introspector instead of raising UnboundLocalError, preserves semantic cache entries, and leaves the output ready to rebuild filesystem nodes on the next file extraction.

Tests

• python -m pytest tests/ -q --tb=short — 3420 passed, 33 skipped
• python -m pytest tests/test_extract_cli.py -q --tb=short — 25 passed
• python -m ruff check graphify/cli.py tests/test_extract_cli.py
• all five tools.skillgen checks
• graphify update .

Closes #2030.