feat: add iOS support and harden receipt scanning

app/ios/ScanReceipts/Features/Scan/ScanViewModel.swift

@@ -0,0 +1,49 @@
+import Foundation
+import UIKit
+
+/// Drives the "Beleg scannen" flow: takes a captured/picked `UIImage`,
+/// uploads it to `POST /api/scan` for AI extraction, and hands the resulting
+/// `Receipt` back to the view. Does NOT persist anything itself — saving
+/// happens later in `ReceiptReviewView` via `ReceiptsAPI.sync(...)`.
+@MainActor
+final class ScanViewModel: ObservableObject {
+    @Published var isUploading = false
+    @Published var errorMessage: String?
+
+    /// Converts the image to JPEG and uploads it for extraction.
+    /// Returns the extracted `Receipt` on success, or `nil` on failure
+    /// (in which case `errorMessage` is set to German user-facing text).
+    func uploadAndExtract(image: UIImage) async -> Receipt? {
+        guard let data = image.jpegData(compressionQuality: 0.85) else {
+            errorMessage = "Bild konnte nicht verarbeitet werden."
+            return nil
+        }
+        return await uploadAndExtract(imageData: data)
+    }
+
+    /// Same as above, but takes already-encoded JPEG data directly.
+    func uploadAndExtract(imageData: Data) async -> Receipt? {
+        isUploading = true
+        errorMessage = nil
+        defer { isUploading = false }
+
+        do {
+            let response = try await ReceiptsAPI.scan(
+                fileData: imageData,
+                fileName: "receipt.jpg",
+                mimeType: "image/jpeg"
+            )
+            guard let receipt = response.receipt else {
+                errorMessage = "Es konnten keine Belegdaten erkannt werden."
+                return nil
+            }
+            return receipt
+        } catch let error as APIError {
+            errorMessage = error.localizedDescription
+            return nil
+        } catch {
+            errorMessage = error.localizedDescription
+            return nil
+        }
+    }
+}