Production readiness hardening and ops tooling

migrations/003_export_job_storage.sql

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+ALTER TABLE export_jobs
+  ADD COLUMN IF NOT EXISTS result_storage_path TEXT,
+  ADD COLUMN IF NOT EXISTS result_size_bytes INTEGER,
+  ADD COLUMN IF NOT EXISTS expires_at TIMESTAMPTZ;
+
+CREATE INDEX IF NOT EXISTS idx_export_jobs_expires_at ON export_jobs (expires_at);