feat(schedule): support random cover source for scheduled and manual publish

Add a cover mode to schedule tasks so auto-publish can pick a random cover
image instead of a fixed one, scoped to all images or a specific folder.

- migration: add cover_mode / cover_random_scope / cover_random_folder_id
  columns and check constraints on schedule_tasks
- backend: validate cover mode/scope/folder on create & update; the dispatch
  worker resolves a random active image (signed asset URL) at enqueue time
- frontend: new CoverSourceSelector component wired into GenerateTaskDrawer
  and PublishArticleModal, plus coverSource i18n strings

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/migrations/20260614100000_add_schedule_random_cover.down.sql

@@ -0,0 +1,9 @@
+ALTER TABLE schedule_tasks
+    DROP CONSTRAINT IF EXISTS chk_schedule_tasks_cover_random_folder_id,
+    DROP CONSTRAINT IF EXISTS chk_schedule_tasks_cover_random_scope,
+    DROP CONSTRAINT IF EXISTS chk_schedule_tasks_cover_mode;
+
+ALTER TABLE schedule_tasks
+    DROP COLUMN IF EXISTS cover_random_folder_id,
+    DROP COLUMN IF EXISTS cover_random_scope,
+    DROP COLUMN IF EXISTS cover_mode;

server/migrations/20260614100000_add_schedule_random_cover.up.sql

@@ -0,0 +1,19 @@
+ALTER TABLE schedule_tasks
+    ADD COLUMN cover_mode VARCHAR(20) NOT NULL DEFAULT 'specific',
+    ADD COLUMN cover_random_scope VARCHAR(20) NOT NULL DEFAULT 'all',
+    ADD COLUMN cover_random_folder_id BIGINT;
+
+ALTER TABLE schedule_tasks
+    ADD CONSTRAINT chk_schedule_tasks_cover_mode
+        CHECK (cover_mode IN ('specific', 'random'));
+
+ALTER TABLE schedule_tasks
+    ADD CONSTRAINT chk_schedule_tasks_cover_random_scope
+        CHECK (cover_random_scope IN ('all', 'folder'));
+
+ALTER TABLE schedule_tasks
+    ADD CONSTRAINT chk_schedule_tasks_cover_random_folder_id
+        CHECK (
+            cover_random_folder_id IS NULL
+            OR cover_random_folder_id > 0
+        );