Add versioning, update tool, and soft-delete

Schema: public_id + version columns with UNIQUE constraint,
delete_timestamp for soft-delete.

Tools: store_flashcard accepts optional public_id, update_flashcard
creates a new version preserving history, delete_flashcard inserts
a tombstone row and removes the Qdrant point.

Qdrant: point ID is public_id (only latest version indexed).

.env.example

@@ -7,4 +7,4 @@ OPENAI_API_KEY=sk-...
 EMBEDDING_MODEL=text-embedding-3-small
 MCP_TOKEN=change-me
 PORT=8080
-SQLITE_PATH=knowledge.db
+SQLITE_PATH=flashcard.db