feat(oss): make spaCy an optional dependency

Move spacy from core dependencies to optional [nlp] extra.
The system gracefully degrades without spaCy:
- Lemmatization falls back to raw text
- Entity extraction returns empty (no entity boost)
- BM25 keyword search still works on raw text
- Semantic search and additive scoring unaffected

Install with NLP support: pip install mem0ai[nlp]

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -90,6 +90,13 @@ Install the sdk via pip:
 pip install mem0ai
 ```
 
+For enhanced hybrid search with BM25 keyword matching and entity extraction, install with NLP support:
+
+```bash
+pip install mem0ai[nlp]
+python -m spacy download en_core_web_sm
+```
+
 Install sdk via npm:
 ```bash
 npm install mem0ai