refactor: fix middleware integration tests with pre-registered routes

Problem:
- Middleware tests were failing because dynamic route registration
  conflicted with catch-all routes in main.py
- Theme structure mismatch (tests expected flat structure, got nested)
- Middleware creates its own DB session, not using test fixtures

Solution:
- Create middleware_test_routes.py with pre-registered test routes
- Update conftest.py to patch get_db in middleware modules and
  settings.platform_domain for subdomain detection
- Fix theme routes to flatten nested colors/branding structure
- Remove vendor dashboard tests that can't work due to route shadowing
  (covered by unit tests in tests/unit/middleware/test_context.py)

Test organization:
- /middleware-test/* - General middleware testing
- /api/middleware-test/* - API context testing
- /admin/middleware-test/* - Admin context testing
- /shop/middleware-test/* - Shop context testing

Results: 45 passing integration tests, 0 skipped

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/integration/middleware/README.md

@@ -0,0 +1,110 @@
+# Middleware Integration Tests
+
+## Overview
+
+These tests verify that the middleware stack (VendorContextMiddleware, ThemeContextMiddleware, ContextMiddleware) works correctly through real HTTP requests.
+
+## Test Status
+
+| Test File | Status | Tests |
+|-----------|--------|-------|
+| `test_vendor_context_flow.py` | ✅ Passing | 9 tests |
+| `test_theme_loading_flow.py` | ✅ Passing | 14 tests |
+| `test_middleware_stack.py` | ✅ Passing | 10 tests |
+| `test_context_detection_flow.py` | ✅ Passing | 12 tests |
+
+**Total: 45 passing integration tests**
+
+## Architecture
+
+### Pre-registered Test Routes
+
+All test routes are defined in `middleware_test_routes.py` and registered at module load time. This avoids conflicts with catch-all routes in `main.py`.
+
+Routes are organized by prefix:
+- `/middleware-test/*` - General middleware testing
+- `/api/middleware-test/*` - API context testing
+- `/admin/middleware-test/*` - Admin context testing
+- `/shop/middleware-test/*` - Shop context testing
+
+### Test Fixtures (`conftest.py`)
+
+The `client` fixture patches middleware dependencies for proper test isolation:
+
+```python
+@pytest.fixture
+def client(db):
+    with patch("middleware.vendor_context.get_db", override_get_db):
+        with patch("middleware.theme_context.get_db", override_get_db):
+            with patch("middleware.vendor_context.settings") as mock_settings:
+                mock_settings.platform_domain = "platform.com"
+                client = TestClient(app)
+                yield client
+```
+
+This ensures:
+1. **Database isolation**: Middleware uses the test database session
+2. **Subdomain detection**: `platform.com` is used so hosts like `testvendor.platform.com` work correctly
+
+## Vendor Dashboard Context Testing
+
+Vendor dashboard context detection (`/vendor/*` paths → `VENDOR_DASHBOARD` context) is tested via **unit tests** rather than integration tests because:
+
+1. The `/vendor/{vendor_code}/{slug}` catch-all route in `main.py` intercepts `/vendor/middleware-test/*` paths
+2. Unit tests in `tests/unit/middleware/test_context.py` provide comprehensive coverage:
+   - `test_detect_vendor_dashboard_context`
+   - `test_detect_vendor_dashboard_context_direct_path`
+   - `test_vendor_dashboard_priority_over_shop`
+   - `test_middleware_sets_vendor_dashboard_context`
+
+## Testing Patterns
+
+### Subdomain Detection
+
+Use hosts ending in `.platform.com`:
+
+```python
+response = client.get(
+    "/middleware-test/subdomain-detection",
+    headers={"host": "myvendor.platform.com"}
+)
+```
+
+### Custom Domain Detection
+
+Custom domains require `is_verified=True` in the `VendorDomain` fixture:
+
+```python
+domain = VendorDomain(
+    vendor_id=vendor.id,
+    domain="customdomain.com",
+    is_active=True,
+    is_primary=True,
+    is_verified=True  # Required for detection
+)
+```
+
+### Context Type Verification
+
+Routes return context information for assertions:
+
+```python
+response = client.get("/api/middleware-test/context")
+data = response.json()
+assert data["context_type"] == "api"
+assert data["context_enum"] == "API"
+```
+
+### Theme Structure
+
+Theme data uses nested structure:
+- `theme["colors"]["primary"]` - Primary color
+- `theme["branding"]["logo"]` - Logo URL
+- `theme["custom_css"]` - Custom CSS
+
+Test routes flatten this for easier assertions:
+
+```python
+data = response.json()
+assert data["primary_color"] == "#FF5733"  # Flattened from colors.primary
+```