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>

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."""
-    import spacy
+    """Download en_core_web_sm if spaCy is installed but model is missing."""
+    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...")