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/innercontext/api/products.py

@@ -188,7 +188,7 @@ def get_or_404(session: Session, model, record_id) -> object:
 # ---------------------------------------------------------------------------
 
 
-@router.get("/", response_model=list[Product])
+@router.get("", response_model=list[Product])
 def list_products(
     category: Optional[ProductCategory] = None,
     brand: Optional[str] = None,
@@ -224,7 +224,7 @@ def list_products(
     return products
 
 
-@router.post("/", response_model=Product, status_code=201)
+@router.post("", response_model=Product, status_code=201)
 def create_product(data: ProductCreate, session: Session = Depends(get_session)):
     product = Product(
         id=uuid4(),
@@ -236,9 +236,13 @@ def create_product(data: ProductCreate, session: Session = Depends(get_session))
     return product
 
 
-@router.get("/{product_id}", response_model=Product)
+@router.get("/{product_id}")
 def get_product(product_id: UUID, session: Session = Depends(get_session)):
-    return get_or_404(session, Product, product_id)
+    product = get_or_404(session, Product, product_id)
+    inventory = session.exec(select(ProductInventory).where(ProductInventory.product_id == product_id)).all()
+    data = product.model_dump(mode="json")
+    data["inventory"] = [item.model_dump(mode="json") for item in inventory]
+    return data
 
 
 @router.patch("/{product_id}", response_model=Product)