Refactor backend database connection, migration, seed

backend/migrations/20251017065539_00004_create_selections_table.sql

@@ -0,0 +1,17 @@
+-- +goose Up
+-- +goose StatementBegin
+CREATE TABLE IF NOT EXISTS selections (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id    INTEGER NOT NULL,
+    main       TEXT NOT NULL,
+    side       TEXT NOT NULL,
+    soup       TEXT,
+    created_at TEXT NOT NULL,
+    FOREIGN KEY (user_id) REFERENCES users(id)
+)
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+DROP TABLE IF EXISTS selections;
+-- +goose StatementEnd