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>

pyproject.toml

@@ -21,10 +21,12 @@ dependencies = [
     "pytz>=2024.1",
     "sqlalchemy>=2.0.31",
     "protobuf>=5.29.0,<6.0.0",
-    "spacy>=3.7.0",
 ]
 
 [project.optional-dependencies]
+nlp = [
+    "spacy>=3.7.0",
+]
 graph = [
     "langchain-neo4j>=0.4.0",
     "langchain-aws>=0.2.23",