feat: Add database import/export functionality (Issue #10)

Backend:
- New /api/v1/backup/export endpoint - exports all data as single JSON
- New /api/v1/backup/import endpoint - imports JSON with merge or replace
- Handles vendors, filaments, spools, adjustments, settings
- Remaps IDs to maintain relationships during import

Frontend:
- Added Backup & Restore section to Settings page
- Download Backup button for export
- Import with merge or replace options
- Warning confirmation for replace mode

This enables migrating between PostgreSQL and SQLite databases.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/src/pages/settings/generalSettings.tsx

@@ -1,6 +1,9 @@
+import { DownloadOutlined, UploadOutlined } from "@ant-design/icons";
 import { useTranslate } from "@refinedev/core";
-import { Button, Checkbox, Form, Input, message } from "antd";
-import { useEffect } from "react";
+import { Button, Checkbox, Divider, Form, Input, message, Popconfirm, Space, Upload } from "antd";
+import type { UploadFile } from "antd";
+import { useEffect, useState } from "react";
+import { getAPIURL } from "../../utils/url";
 import { useGetSettings, useSetSetting } from "../../utils/querySettings";
 
 export function GeneralSettings() {
@@ -110,7 +113,141 @@ export function GeneralSettings() {
           </Button>
         </Form.Item>
       </Form>
+
+      <Divider />
+
+      <BackupRestore messageApi={messageApi} t={t} />
+
       {contextHolder}
     </>
   );
 }
+
+interface BackupRestoreProps {
+  messageApi: ReturnType<typeof message.useMessage>[0];
+  t: ReturnType<typeof useTranslate>;
+}
+
+function BackupRestore({ messageApi, t }: BackupRestoreProps) {
+  const [importing, setImporting] = useState(false);
+  const [fileList, setFileList] = useState<UploadFile[]>([]);
+
+  const handleExport = async () => {
+    try {
+      const response = await fetch(`${getAPIURL()}/backup/export`);
+      if (!response.ok) throw new Error("Export failed");
+
+      const blob = await response.blob();
+      const filename = response.headers.get("content-disposition")?.match(/filename="(.+)"/)?.[1]
+        || `spoolman_backup_${new Date().toISOString().slice(0, 10)}.json`;
+
+      const url = window.URL.createObjectURL(blob);
+      const a = document.createElement("a");
+      a.href = url;
+      a.download = filename;
+      document.body.appendChild(a);
+      a.click();
+      window.URL.revokeObjectURL(url);
+      document.body.removeChild(a);
+
+      messageApi.success(t("settings.backup.export_success"));
+    } catch (error) {
+      messageApi.error(t("settings.backup.export_error"));
+    }
+  };
+
+  const handleImport = async (replace: boolean) => {
+    if (fileList.length === 0) {
+      messageApi.warning(t("settings.backup.select_file"));
+      return;
+    }
+
+    setImporting(true);
+    try {
+      const formData = new FormData();
+      formData.append("file", fileList[0].originFileObj as Blob);
+
+      const response = await fetch(`${getAPIURL()}/backup/import?replace=${replace}`, {
+        method: "POST",
+        body: formData,
+      });
+
+      const result = await response.json();
+
+      if (result.success) {
+        messageApi.success(
+          `${t("settings.backup.import_success")}: ${result.vendors_imported} vendors, ${result.filaments_imported} filaments, ${result.spools_imported} spools`
+        );
+        setFileList([]);
+        // Reload the page to refresh all data
+        window.location.reload();
+      } else {
+        messageApi.error(`${t("settings.backup.import_error")}: ${result.errors.join(", ")}`);
+      }
+    } catch (error) {
+      messageApi.error(t("settings.backup.import_error"));
+    } finally {
+      setImporting(false);
+    }
+  };
+
+  return (
+    <div style={{ maxWidth: "600px", margin: "0 auto" }}>
+      <h3>{t("settings.backup.title")}</h3>
+      <p style={{ marginBottom: "1em", color: "#888" }}>
+        {t("settings.backup.description")}
+      </p>
+
+      <Space direction="vertical" size="middle" style={{ width: "100%" }}>
+        <div>
+          <h4>{t("settings.backup.export_title")}</h4>
+          <Button icon={<DownloadOutlined />} onClick={handleExport}>
+            {t("settings.backup.export_button")}
+          </Button>
+        </div>
+
+        <div>
+          <h4>{t("settings.backup.import_title")}</h4>
+          <Space direction="vertical" size="small" style={{ width: "100%" }}>
+            <Upload
+              accept=".json"
+              maxCount={1}
+              fileList={fileList}
+              beforeUpload={() => false}
+              onChange={({ fileList }) => setFileList(fileList)}
+            >
+              <Button icon={<UploadOutlined />}>{t("settings.backup.select_file")}</Button>
+            </Upload>
+
+            <Space>
+              <Button
+                type="primary"
+                onClick={() => handleImport(false)}
+                loading={importing}
+                disabled={fileList.length === 0}
+              >
+                {t("settings.backup.import_merge")}
+              </Button>
+
+              <Popconfirm
+                title={t("settings.backup.replace_warning_title")}
+                description={t("settings.backup.replace_warning")}
+                onConfirm={() => handleImport(true)}
+                okText={t("buttons.yes")}
+                cancelText={t("buttons.no")}
+              >
+                <Button
+                  danger
+                  loading={importing}
+                  disabled={fileList.length === 0}
+                >
+                  {t("settings.backup.import_replace")}
+                </Button>
+              </Popconfirm>
+            </Space>
+          </Space>
+        </div>
+      </Space>
+    </div>
+  );
+}