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/003_create_recipes.sql

@@ -3,7 +3,7 @@ CREATE TYPE recipe_source     AS ENUM ('spoonacular', 'ai', 'user');
 CREATE TYPE recipe_difficulty AS ENUM ('easy', 'medium', 'hard');
 
 CREATE TABLE recipes (
-    id             UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    id             UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     source         recipe_source NOT NULL DEFAULT 'spoonacular',
     spoonacular_id INTEGER UNIQUE,