Pin spaCy + en_core_web_sm; restore entity/event/place layer

[StigNorland]

spaCy is not installed and is absent from requirements.txt, but context.py depends on it for all L0 GROUND entities, events, and place expressions. Without it, _get_nlp() returns None and those extractors silently return [] — degrading results instead of erroring.

• Add spacy to requirements.txt
• Document/install the en_core_web_sm model
• Verify entity/event/place nodes appear after a sample ingest

Refs: src/library/context.py:36 (_get_nlp), :54 (_extract_entities_spacy)