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>

drizzle/meta/_journal.json

@@ -8,6 +8,13 @@
       "when": 1768462458852,
       "tag": "0000_burly_unus",
       "breakpoints": true
+    },
+    {
+      "idx": 1,
+      "version": "6",
+      "when": 1768641501799,
+      "tag": "0001_free_hiroim",
+      "breakpoints": true
     }
   ]
 }