Add user dashboard and per-user food item ownership

Introduces a user dashboard route and template, moving dashboard logic to a user blueprint. FoodItem now has an owner_id field and a unique constraint on (barcode, owner_id), with relationships set up in the User model. Updates food item creation to associate with the current user, and adds a utility script for dropping a temporary table.

application/user/routes.py

@@ -0,0 +1,20 @@
+from flask import Blueprint, redirect, url_for, render_template
+from flask_login import current_user
+
+user_bp = Blueprint(
+    "user",
+    __name__,
+    template_folder="templates",
+)
+
+
+@user_bp.before_request
+def login_required():
+    if not current_user.is_authenticated:
+        return redirect(url_for("login"))
+
+
+@user_bp.route("/dashboard", methods=["GET"])
+def dashboard():
+    items = current_user.food_items.all()
+    return render_template("dashboard.html", items=items)