Split domain processing into separate check and crawl loops

- StartDomainCheckLoop: DNS verification for unchecked domains (1000 workers)
- StartFeedCrawlLoop: Feed discovery on DNS-verified domains (100 workers)

This fixes starvation where 104M unchecked domains blocked 1.2M
DNS-verified domains from ever being crawled for feeds.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

domain.go

@@ -141,19 +141,38 @@ func (c *Crawler) getDomain(host string) (*Domain, error) {
 	return domain, nil
 }
 
-// GetDomainsToProcess returns domains needing processing (domain_check or feed_crawl)
-// crawled_at = zero time means needs domain_check, +1 sec means needs feed_crawl
-// Domains with errors are retried when crawled_at < now (scheduled by ErrorRetryDelay)
-func (c *Crawler) GetDomainsToProcess(limit int) ([]*Domain, error) {
+// GetDomainsToCheck returns unchecked domains needing DNS lookup (domain_check)
+// crawled_at = zero time means needs domain_check
+func (c *Crawler) GetDomainsToCheck(limit int) ([]*Domain, error) {
+	rows, err := c.db.Query(`
+		SELECT host, status, crawled_at, feeds_found, last_error, tld
+		FROM domains
+		WHERE status = 'pass'
+			AND crawled_at = '0001-01-01 00:00:00'
+			AND last_error IS NULL
+		LIMIT $1
+	`, limit)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	return c.scanDomains(rows)
+}
+
+// GetDomainsToCrawl returns DNS-verified domains needing feed discovery (feed_crawl)
+// crawled_at = +1 sec means passed DNS check, ready for crawl
+// Also includes domains with errors that are due for retry
+func (c *Crawler) GetDomainsToCrawl(limit int) ([]*Domain, error) {
 	now := time.Now()
 	rows, err := c.db.Query(`
 		SELECT host, status, crawled_at, feeds_found, last_error, tld
 		FROM domains
 		WHERE status = 'pass' AND (
-			(crawled_at < '0001-01-02' AND last_error IS NULL)  -- new domains
-			OR (crawled_at < $1 AND last_error IS NOT NULL)     -- retry errors after delay
+			(crawled_at = '0001-01-01 00:00:01' AND last_error IS NULL)  -- passed DNS, ready to crawl
+			OR (crawled_at < $1 AND crawled_at > '0001-01-01 00:00:01' AND last_error IS NOT NULL)  -- retry errors
 		)
-		ORDER BY crawled_at ASC
+		ORDER BY last_error IS NULL DESC, crawled_at ASC
 		LIMIT $2
 	`, now, limit)
 	if err != nil {