feat: prepare database and UI for DCL integration

Add infrastructure for future DARC Community Logbook (DCL) integration:
- Database schema: Add dcl_api_key, my_darc_dok, darc_dok, dcl_qsl_rdate, dcl_qsl_rstatus fields
- Create DCL service stub with placeholder functions for when DCL provides API
- Backend API: Add /api/auth/dcl-credentials endpoint for API key management
- Frontend settings: Add DCL API key input with informational notice about API availability
- QSO table: Add My DOK and DOK columns, update confirmation column for multiple services

Note: DCL download API is not yet available. These changes prepare the application
for future implementation when DCL adds programmatic access.

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

src/backend/index.js

@@ -7,6 +7,7 @@ import {
   authenticateUser,
   getUserById,
   updateLoTWCredentials,
+  updateDCLCredentials,
 } from './services/auth.service.js';
 import {
   getUserQSOs,
@@ -235,6 +236,40 @@ const app = new Elysia()
     }
   )
 
+  /**
+   * PUT /api/auth/dcl-credentials
+   * Update DCL credentials (requires authentication)
+   */
+  .put(
+    '/api/auth/dcl-credentials',
+    async ({ user, body, set }) => {
+      if (!user) {
+        set.status = 401;
+        return { success: false, error: 'Unauthorized' };
+      }
+
+      try {
+        await updateDCLCredentials(user.id, body.dclApiKey);
+
+        return {
+          success: true,
+          message: 'DCL credentials updated successfully',
+        };
+      } catch (error) {
+        set.status = 500;
+        return {
+          success: false,
+          error: 'Failed to update DCL credentials',
+        };
+      }
+    },
+    {
+      body: t.Object({
+        dclApiKey: t.String(),
+      }),
+    }
+  )
+
   /**
    * POST /api/lotw/sync
    * Queue a LoTW sync job (requires authentication)