feat: add vote_flashcard MCP tool

migrations/000003_add_vote.sql

@@ -0,0 +1,14 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS vote (
+    _id INTEGER PRIMARY KEY,
+    flashcard_id INTEGER NOT NULL REFERENCES flashcard(_id),
+    value INTEGER NOT NULL CHECK(value >= -3 AND value <= 3),
+    "user" TEXT NOT NULL,
+    repo TEXT NOT NULL,
+    message TEXT NOT NULL,
+    created TEXT NOT NULL DEFAULT (datetime('now')),
+    UNIQUE(flashcard_id, "user", repo)
+);
+
+-- +goose Down
+DROP TABLE IF EXISTS vote;