fix: resolve frontend/backend integration bugs

- Rename skincare route prefix /skin-snapshots → /skincare to match API client
- Add redirect_slashes=False to FastAPI app; change collection routes from "/" to ""
  to eliminate 307 redirects on POST/GET without trailing slash
- Fix redirect() inside try/catch in products/new and routines/new server actions
  (SvelteKit redirect() throws and was being caught as a 500 error)
- Eagerly load inventory and steps relationships via explicit SELECT + model_dump(mode="json"),
  working around SQLModel 0.0.37 not serializing Relationship fields in response_model
- Add field_validator for product_effect_profile to coerce DB-returned dict → ProductEffectProfile,
  eliminating Pydantic serializer warning
- Update all tests to use routes without trailing slash

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

backend/tests/conftest.py

@@ -61,7 +61,7 @@ def product_data():
 
 @pytest.fixture()
 def created_product(client, product_data):
-    r = client.post("/products/", json=product_data)
+    r = client.post("/products", json=product_data)
     assert r.status_code == 201
     return r.json()
 
@@ -81,7 +81,7 @@ def created_medication(client, medication_data):
 @pytest.fixture()
 def created_routine(client):
     r = client.post(
-        "/routines/", json={"routine_date": "2026-02-26", "part_of_day": "am"}
+        "/routines", json={"routine_date": "2026-02-26", "part_of_day": "am"}
     )
     assert r.status_code == 201
     return r.json()