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/services/auth.service.js

@@ -126,3 +126,19 @@ export async function updateLoTWCredentials(userId, lotwUsername, lotwPassword)
     })
     .where(eq(users.id, userId));
 }
+
+/**
+ * Update user's DCL API key
+ * @param {number} userId - User ID
+ * @param {string} dclApiKey - DCL API key
+ * @returns {Promise<void>}
+ */
+export async function updateDCLCredentials(userId, dclApiKey) {
+  await db
+    .update(users)
+    .set({
+      dclApiKey,
+      updatedAt: new Date(),
+    })
+    .where(eq(users.id, userId));
+}