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

mem0/utils/spacy_models.py

@@ -19,8 +19,13 @@ _lock = threading.Lock()
 
 
 def _ensure_model_available():
-    """Download en_core_web_sm if not already installed. Does not load the model."""
+    """Download en_core_web_sm if spaCy is installed but model is missing."""
-    import spacy
+    try:
+        import spacy
+    except ImportError:
+        raise ImportError(
+            "spaCy is not installed. Install it with: pip install mem0ai[nlp]"
+        )
 
     if not spacy.util.is_package("en_core_web_sm"):
         logger.info("Downloading spaCy model en_core_web_sm...")

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",