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/001_create_users.sql

@@ -1,5 +1,35 @@
 -- +goose Up
-CREATE EXTENSION IF NOT EXISTS "uuid-ossp";
+
+-- Generate UUID v7 (time-ordered, millisecond precision).
+-- Structure: 48-bit unix_ts_ms | 4-bit version (0111) | 12-bit rand_a | 2-bit variant (10) | 62-bit rand_b
+CREATE OR REPLACE FUNCTION uuid_generate_v7()
+RETURNS uuid
+AS $$
+DECLARE
+    unix_ts_ms bytea;
+    uuid_bytes bytea;
+BEGIN
+    -- 48-bit Unix timestamp in milliseconds.
+    -- int8send produces 8 big-endian bytes; skip the first 2 zero bytes to get 6.
+    unix_ts_ms = substring(int8send(floor(extract(epoch from clock_timestamp()) * 1000)::bigint) from 3);
+
+    -- Use a random v4 UUID as the source of random bits for rand_a and rand_b.
+    uuid_bytes = uuid_send(gen_random_uuid());
+
+    -- Overwrite bytes 0–5 with the timestamp (positions 1–6 in 1-indexed bytea).
+    uuid_bytes = overlay(uuid_bytes placing unix_ts_ms from 1 for 6);
+
+    -- Set version nibble (bits 48–51) to 0111 (7).
+    uuid_bytes = set_bit(uuid_bytes, 48, 0);
+    uuid_bytes = set_bit(uuid_bytes, 49, 1);
+    uuid_bytes = set_bit(uuid_bytes, 50, 1);
+    uuid_bytes = set_bit(uuid_bytes, 51, 1);
+
+    -- Variant bits (64–65) stay at 10 as inherited from gen_random_uuid().
+
+    RETURN encode(uuid_bytes, 'hex')::uuid;
+END
+$$ LANGUAGE plpgsql VOLATILE;
 
 CREATE TYPE user_plan AS ENUM ('free', 'paid');
 CREATE TYPE user_gender AS ENUM ('male', 'female');
@@ -7,7 +37,7 @@ CREATE TYPE user_goal AS ENUM ('lose', 'maintain', 'gain');
 CREATE TYPE activity_level AS ENUM ('low', 'moderate', 'high');
 
 CREATE TABLE users (
-    id            UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    id            UUID PRIMARY KEY DEFAULT uuid_generate_v7(),
     firebase_uid  VARCHAR(128) NOT NULL UNIQUE,
     email         VARCHAR(255) NOT NULL,
     name          VARCHAR(255) NOT NULL DEFAULT '',
@@ -47,3 +77,4 @@ DROP TYPE IF EXISTS activity_level;
 DROP TYPE IF EXISTS user_goal;
 DROP TYPE IF EXISTS user_gender;
 DROP TYPE IF EXISTS user_plan;
+DROP FUNCTION IF EXISTS uuid_generate_v7();

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,

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,
 

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,

backend/migrations/006_create_products.sql

@@ -1,6 +1,6 @@
 -- +goose Up
 CREATE TABLE products (
-    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,
     mapping_id   UUID REFERENCES ingredient_mappings(id),
     name         TEXT NOT NULL,

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 '[]',

backend/migrations/008_create_meal_diary.sql

@@ -1,7 +1,7 @@
 -- +goose Up
 
 CREATE TABLE meal_diary (
-    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,
     date       DATE NOT NULL,
     meal_type  TEXT NOT NULL,