feat: replace UUID v4 with UUID v7 across backend

Define uuid_generate_v7() in the first migration and switch all table
primary key defaults to it. Remove the uuid-ossp extension dependency.
Update refresh token and request ID generation in Go code to use
uuid.NewV7() from the existing google/uuid v1.6.0 library.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/007_create_menu_plans.sql

@@ -1,7 +1,7 @@
 -- +goose Up
 
 CREATE TABLE menu_plans (
-    id         UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    id         UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     user_id    UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
     week_start DATE NOT NULL,
     created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
@@ -9,7 +9,7 @@ CREATE TABLE menu_plans (
 );
 
 CREATE TABLE menu_items (
-    id           UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    id           UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     menu_plan_id UUID NOT NULL REFERENCES menu_plans(id) ON DELETE CASCADE,
     day_of_week  INT  NOT NULL CHECK (day_of_week BETWEEN 1 AND 7),
     meal_type    TEXT NOT NULL CHECK (meal_type IN ('breakfast','lunch','dinner')),
@@ -21,7 +21,7 @@ CREATE TABLE menu_items (
 -- Stores the generated shopping list for a menu plan.
 -- items is a JSONB array of {name, category, amount, unit, checked, in_stock}.
 CREATE TABLE shopping_lists (
-    id           UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    id           UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     user_id      UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
     menu_plan_id UUID REFERENCES menu_plans(id) ON DELETE CASCADE,
     items        JSONB NOT NULL DEFAULT '[]',