[Feature] Per-topic/per-author subscriptions with scheduled digests

[jerry609]

Source TODO

• SubscriptionModel
• CRUD APIs for subscriptions
• ARQ cron generation
• Incremental new-paper detection

Scope

• Model + migration + APIs
• Scheduler and job execution path
• Dedup/new-match logic

Acceptance Criteria

• User receives only new matching results per subscription

[jerry609]

Implemented in commit 4cee232: added subscription model+migration, CRUD/run APIs, ARQ cron/job, and incremental new-match push flow.