feat: add websocket support for real-time scrape job monitoring

migrations/20260129183411_add_queued_at_to_scrape_jobs.sql

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+-- Add queued_at column to track when a job last entered the "ready to pick up" state.
+-- For fresh jobs this equals execute_at; for retried jobs it is updated to NOW().
+ALTER TABLE scrape_jobs
+    ADD COLUMN queued_at TIMESTAMPTZ NOT NULL DEFAULT NOW();
+
+-- Backfill existing rows: set queued_at = execute_at (best approximation)
+UPDATE scrape_jobs SET queued_at = execute_at;