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/skincare.py

@@ -78,7 +78,7 @@ def get_or_404(session: Session, model, record_id) -> object:
 # ---------------------------------------------------------------------------
 
 
-@router.get("/", response_model=list[SkinConditionSnapshot])
+@router.get("", response_model=list[SkinConditionSnapshot])
 def list_snapshots(
     from_date: Optional[date] = None,
     to_date: Optional[date] = None,
@@ -95,7 +95,7 @@ def list_snapshots(
     return session.exec(stmt).all()
 
 
-@router.post("/", response_model=SkinConditionSnapshot, status_code=201)
+@router.post("", response_model=SkinConditionSnapshot, status_code=201)
 def create_snapshot(
     data: SnapshotCreate, session: Session = Depends(get_session)
 ):