feat: production routing support for subdomain and custom domain modes

Double-mount store routes at /store/* and /store/{store_code}/* so the
same handlers work in dev path-based, prod path-based, prod subdomain,
and prod custom-domain modes.  Wire StorePlatform.custom_subdomain into
StoreContextMiddleware for per-platform subdomain overrides.  Add admin
custom-domain management UI, fix stale /shop/ reset link, add
/merchants/ to reserved paths, and server-render window.STORE_CODE for
JS that previously parsed the URL.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/api/test_get_resolved_store_code.py

@@ -0,0 +1,119 @@
+# tests/unit/api/test_get_resolved_store_code.py
+"""
+Unit tests for get_resolved_store_code dependency.
+
+Tests the store code resolution logic that supports both:
+- Path-based routing: /store/{store_code}/dashboard (store_code in URL)
+- Subdomain/custom domain: /store/dashboard (store resolved by middleware)
+"""
+
+from unittest.mock import MagicMock, Mock
+
+import pytest
+from fastapi import HTTPException
+
+from app.api.deps import get_resolved_store_code
+
+
+@pytest.mark.unit
+@pytest.mark.stores
+class TestGetResolvedStoreCode:
+    """Test suite for get_resolved_store_code dependency."""
+
+    @pytest.mark.asyncio
+    async def test_returns_store_code_from_path_params(self):
+        """When store_code is in path params (path-based routing), return it."""
+        request = Mock()
+        request.path_params = {"store_code": "ACME"}
+        request.state = MagicMock()
+
+        result = await get_resolved_store_code(request)
+
+        assert result == "ACME"
+
+    @pytest.mark.asyncio
+    async def test_returns_store_code_from_middleware_state(self):
+        """When no path param but middleware resolved store, return store.store_code."""
+        mock_store = Mock()
+        mock_store.store_code = "ORION"
+
+        request = Mock()
+        request.path_params = {}
+        request.state = MagicMock()
+        request.state.store = mock_store
+
+        result = await get_resolved_store_code(request)
+
+        assert result == "ORION"
+
+    @pytest.mark.asyncio
+    async def test_path_param_takes_priority_over_middleware(self):
+        """Path param should be preferred even if middleware also resolved a store."""
+        mock_store = Mock()
+        mock_store.store_code = "MIDDLEWARE_STORE"
+
+        request = Mock()
+        request.path_params = {"store_code": "PATH_STORE"}
+        request.state = MagicMock()
+        request.state.store = mock_store
+
+        result = await get_resolved_store_code(request)
+
+        assert result == "PATH_STORE"
+
+    @pytest.mark.asyncio
+    async def test_raises_404_when_no_store_found(self):
+        """When neither path param nor middleware provides store, raise 404."""
+        request = Mock()
+        request.path_params = {}
+        request.state = MagicMock()
+        request.state.store = None
+
+        with pytest.raises(HTTPException) as exc_info:
+            await get_resolved_store_code(request)
+
+        assert exc_info.value.status_code == 404
+        assert "Store not found" in str(exc_info.value.detail)
+
+    @pytest.mark.asyncio
+    async def test_empty_string_path_param_falls_through(self):
+        """Empty string path param should fall through to middleware."""
+        mock_store = Mock()
+        mock_store.store_code = "FALLBACK"
+
+        request = Mock()
+        request.path_params = {"store_code": ""}
+        request.state = MagicMock()
+        request.state.store = mock_store
+
+        result = await get_resolved_store_code(request)
+
+        assert result == "FALLBACK"
+
+    @pytest.mark.asyncio
+    async def test_no_path_params_key_falls_through(self):
+        """When path_params doesn't contain store_code, fall through to middleware."""
+        mock_store = Mock()
+        mock_store.store_code = "WIDGET"
+
+        request = Mock()
+        request.path_params = {"order_id": "123"}
+        request.state = MagicMock()
+        request.state.store = mock_store
+
+        result = await get_resolved_store_code(request)
+
+        assert result == "WIDGET"
+
+    @pytest.mark.asyncio
+    async def test_raises_404_when_no_state_store_attribute(self):
+        """When request.state has no store attribute at all, raise 404."""
+        request = Mock()
+        request.path_params = {}
+        # Simulate missing attribute
+        request.state = MagicMock(spec=[])
+
+        with pytest.raises(HTTPException) as exc_info:
+            await get_resolved_store_code(request)
+
+        assert exc_info.value.status_code == 404