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>

frontend/src/routes/products/new/+page.server.ts

@@ -26,8 +26,9 @@ export const actions: Actions = {
 			.map((t) => t.trim())
 			.filter(Boolean) ?? [];
 
+		let product;
 		try {
-			const product = await createProduct({
+			product = await createProduct({
 				name,
 				brand,
 				category,
@@ -36,9 +37,9 @@ export const actions: Actions = {
 				leave_on,
 				targets
 			});
-			redirect(303, `/products/${product.id}`);
 		} catch (e) {
 			return fail(500, { error: (e as Error).message });
 		}
+		redirect(303, `/products/${product.id}`);
 	}
 };

frontend/src/routes/routines/new/+page.server.ts

@@ -17,11 +17,12 @@ export const actions: Actions = {
 			return fail(400, { error: 'Date and AM/PM are required' });
 		}
 
+		let routine;
 		try {
-			const routine = await createRoutine({ routine_date, part_of_day, notes: notes || undefined });
-			redirect(303, `/routines/${routine.id}`);
+			routine = await createRoutine({ routine_date, part_of_day, notes: notes || undefined });
 		} catch (e) {
 			return fail(500, { error: (e as Error).message });
 		}
+		redirect(303, `/routines/${routine.id}`);
 	}
 };