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/004_create_saved_recipes.sql

@@ -1,6 +1,6 @@
 -- +goose Up
 CREATE TABLE saved_recipes (
-    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,
     title           TEXT NOT NULL,
     description     TEXT,