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).

flake.nix

@@ -1,5 +1,5 @@
 {
-  description = "St-Pierre - MCP Knowledge Server";
+  description = "St-Pierre - MCP Flashcard Server";
 
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
@@ -33,7 +33,7 @@
         in
         {
           options.services.st-pierre = {
-            enable = lib.mkEnableOption "St-Pierre knowledge MCP server";
+            enable = lib.mkEnableOption "St-Pierre flashcard MCP server";
 
             port = lib.mkOption {
               type = lib.types.port;