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).

migrations/000002_add_versioning.sql

@@ -0,0 +1,10 @@
+-- +goose Up
+ALTER TABLE flashcard ADD COLUMN public_id INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE flashcard ADD COLUMN version INTEGER NOT NULL DEFAULT 1;
+UPDATE flashcard SET public_id = _id WHERE public_id = 0;
+CREATE UNIQUE INDEX IF NOT EXISTS idx_flashcard_public_id_version ON flashcard(public_id, version);
+
+-- +goose Down
+DROP INDEX IF EXISTS idx_flashcard_public_id_version;
+ALTER TABLE flashcard DROP COLUMN version;
+ALTER TABLE flashcard DROP COLUMN public_id;