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/0001_free_hiroim.sql

@@ -0,0 +1,18 @@
+CREATE TABLE `sync_jobs` (
+	`id` integer PRIMARY KEY AUTOINCREMENT NOT NULL,
+	`user_id` integer NOT NULL,
+	`status` text NOT NULL,
+	`type` text NOT NULL,
+	`started_at` integer,
+	`completed_at` integer,
+	`result` text,
+	`error` text,
+	`created_at` integer NOT NULL,
+	FOREIGN KEY (`user_id`) REFERENCES `users`(`id`) ON UPDATE no action ON DELETE no action
+);
+--> statement-breakpoint
+ALTER TABLE `qsos` ADD `my_darc_dok` text;--> statement-breakpoint
+ALTER TABLE `qsos` ADD `darc_dok` text;--> statement-breakpoint
+ALTER TABLE `qsos` ADD `dcl_qsl_rdate` text;--> statement-breakpoint
+ALTER TABLE `qsos` ADD `dcl_qsl_rstatus` text;--> statement-breakpoint
+ALTER TABLE `users` ADD `dcl_api_key` text;