feat(schedule): enhance schedule tasks with web search and batch generation support

Add enable_web_search and generate_count columns to schedule_tasks table.
Refactor schedule task service to publish generation jobs to RabbitMQ
instead of inline execution, supporting batch dispatch and backpressure
awareness via scheduler process.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/migrations/20260413120000_enhance_schedule_tasks.down.sql

@@ -0,0 +1,16 @@
+DROP INDEX IF EXISTS idx_schedule_task_operator;
+
+ALTER TABLE schedule_tasks
+    DROP CONSTRAINT IF EXISTS chk_schedule_tasks_generate_count;
+
+ALTER TABLE schedule_tasks
+    DROP COLUMN IF EXISTS generate_count,
+    DROP COLUMN IF EXISTS enable_web_search,
+    DROP COLUMN IF EXISTS operator_id;
+
+ALTER TABLE schedule_tasks
+    ALTER COLUMN target_platform TYPE VARCHAR(50)
+    USING CASE
+        WHEN target_platform IS NULL THEN NULL
+        ELSE LEFT(target_platform, 50)
+    END;