feat(schedule-tasks): support auto-publish via media accounts

Replace the legacy target_platform string on schedule tasks with a
workspace-aware auto-publish payload (auto_publish + publish_account_ids
JSONB + cover_asset_url + cover_image_asset_id) and migrate the schema,
sqlc queries, generated models, domain struct, ScheduleTaskService DTOs,
and dispatch worker to round-trip the new fields. PromptRuleGeneration
gains a WithPublishJobService hook so executeGeneration can enqueue an
auto-publish job once the article is ready, and worker-generate wires
the publish-job service in. On the admin-web side, extract
PublishArticleModal's account-card builders into a shared
publish-account-cards module, rebuild GenerateTaskDrawer's schedule
mode around account selection plus a CoverPickerModal, and surface the
new "auto publish" column on ScheduleTaskTab. Shared types and i18n
strings cover the new fields.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/migrations/20260430120000_add_schedule_auto_publish.down.sql

@@ -0,0 +1,11 @@
+DROP INDEX IF EXISTS idx_schedule_task_workspace;
+
+ALTER TABLE schedule_tasks
+    DROP CONSTRAINT IF EXISTS chk_schedule_tasks_publish_account_ids_json;
+
+ALTER TABLE schedule_tasks
+    DROP COLUMN IF EXISTS cover_image_asset_id,
+    DROP COLUMN IF EXISTS cover_asset_url,
+    DROP COLUMN IF EXISTS publish_account_ids,
+    DROP COLUMN IF EXISTS auto_publish,
+    DROP COLUMN IF EXISTS workspace_id;