Add .docx parser for knowledge uploads #7

Context

Dashboard and API accept PDF/TXT today; .docx was removed from file pickers until a parser exists.

Scope

• Add .docx extraction (e.g. python-docx or unstructured)
• Wire into knowledge ingestion pipeline
• Unit test with a tiny fixture file
• Re-enable .docx in dashboard upload picker

References

• src/voxforge/modules/knowledge/
• PM tasklist non-goal predecessor in Stage 1