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/000001_create_flashcard.sql

@@ -0,0 +1,18 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS flashcard (
+    _id INTEGER PRIMARY KEY,
+    public_id INTEGER NOT NULL,
+    version INTEGER NOT NULL DEFAULT 1,
+    note TEXT NOT NULL,
+    repos TEXT,
+    host TEXT,
+    tags TEXT,
+    "user" TEXT,
+    context TEXT,
+    delete_timestamp TEXT,
+    created TEXT NOT NULL DEFAULT (datetime('now')),
+    UNIQUE(public_id, version)
+);
+
+-- +goose Down
+DROP TABLE IF EXISTS flashcard;