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/002_create_ingredient_mappings.sql

@@ -1,6 +1,6 @@
 -- +goose Up
 CREATE TABLE ingredient_mappings (
-    id                UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    id                UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     canonical_name    VARCHAR(255) NOT NULL,
     canonical_name_ru VARCHAR(255),
     spoonacular_id    INTEGER UNIQUE,