refactor(api): split Axum handlers into handler/service/repo modules

Keep the REST contract; move domain logic out of route files so checkout, fulfillment, and identity can be reused across customer, shop, and admin surfaces.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/migrations/0009_addresses.sql

@@ -0,0 +1,22 @@
+-- Address book: per-customer saved shipping addresses with a single default.
+-- The default invariant is enforced by the partial unique index; handlers keep
+-- it via transactional unset-then-set.
+
+CREATE TABLE addresses (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users (id) ON DELETE CASCADE,
+    recipient TEXT NOT NULL,
+    phone TEXT NOT NULL,
+    country TEXT NOT NULL,
+    region TEXT NOT NULL DEFAULT '',
+    city TEXT NOT NULL,
+    line1 TEXT NOT NULL,
+    postal_code TEXT NOT NULL DEFAULT '',
+    is_default BOOLEAN NOT NULL DEFAULT FALSE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX addresses_user_idx ON addresses (user_id);
+
+CREATE UNIQUE INDEX addresses_one_default_idx ON addresses (user_id) WHERE is_default;