feat: add iOS support and harden receipt scanning

app/ios/ScanReceipts/Networking/ExportAPI.swift

@@ -0,0 +1,55 @@
+import Foundation
+
+/// `/api/export/{csv,excel,pdf}` — Pro-only (403 `pro_required` for a
+/// free-plan account, surfaced via `APIError.server`). Every route takes the
+/// same request body and returns the raw file with its name in
+/// `Content-Disposition`; present the returned `Data` via `UIActivityViewController`
+/// (share sheet) or write it into a temp file and use `.fileExporter` /
+/// `QLPreviewController` — either is fine, this layer just gets you the bytes.
+enum ExportAPI {
+    enum Format: String {
+        case csv, excel, pdf
+
+        var path: String { "/api/export/\(rawValue)" }
+        var fallbackExtension: String {
+            switch self {
+            case .csv: return "csv"
+            case .excel: return "xlsx"
+            case .pdf: return "pdf"
+            }
+        }
+    }
+
+    struct ExportRequestBody: Encodable {
+        let receipts: [Receipt]
+        let locale: String
+        // Only meaningful for the PDF export (date range on the cover page);
+        // harmless to omit for csv/excel, which ignore unknown fields.
+        var dateFrom: String?
+        var dateTo: String?
+    }
+
+    struct ExportResult {
+        let data: Data
+        let fileName: String
+    }
+
+    /// - Parameters:
+    ///   - receipts: exactly the receipts to include — the backend does not
+    ///     look anything up server-side, it only formats what you send.
+    ///   - locale: `"de"` or `"en"`; anything else falls back to German on
+    ///     the backend, so just always pass one of the two.
+    static func export(
+        _ format: Format,
+        receipts: [Receipt],
+        locale: String = "de",
+        dateFrom: String? = nil,
+        dateTo: String? = nil
+    ) async throws -> ExportResult {
+        let body = ExportRequestBody(receipts: receipts, locale: locale, dateFrom: dateFrom, dateTo: dateTo)
+        let request = try APIRequest.json(path: format.path, method: .post, body: body)
+        let (data, suggestedName) = try await APIClient.shared.sendForFile(request)
+        let fileName = suggestedName ?? "Belege_Export.\(format.fallbackExtension)"
+        return ExportResult(data: data, fileName: fileName)
+    }
+}