feat: data store replication configuration and TLS upgrade (#975)

- Add centralised qdrant_config.py helper with env-var fallback for
  QDRANT_URL, QDRANT_API_KEY, QDRANT_REPLICATION_FACTOR, QDRANT_SHARD_NUMBER
- Update all 6 Qdrant processors to use the helper; writers pass
  replication_factor and shard_number to create_collection
- Fix hardcoded Cassandra replication_factor=1 in cassandra_kg.py,
  write.py, and sparql_cassandra.py to respect CASSANDRA_REPLICATION_FACTOR
- Upgrade Cassandra TLS from deprecated PROTOCOL_TLSv1_2 to
  ssl.create_default_context() across all connectors

tests/unit/test_reliability/test_null_embedding_protection.py

@@ -259,6 +259,8 @@ class TestGraphEmbeddingsNullProtection:
         proc.collection_exists = MagicMock(return_value=True)
         proc._cache_lock = asyncio.Lock()
         proc._known_collections = set()
+        proc.replication_factor = 1
+        proc.shard_number = 1
 
         msg = MagicMock()
         msg.metadata.collection = "graphs"