Improve crawler performance and logging

- Optimize job scheduler to use NumOfWorkers for URL limits
- Clean up verbose logging in worker processing
- Update log messages for better clarity

misc/sql/fetch-snapshot-history.sql

@@ -0,0 +1,6 @@
+select count(*) from snapshots
+  where last_crawled < now() - interval '30 days'
+    and error IS NULL
+    and gemtext IS NOT NULL
+    and mimetype='text/gemini'
+    and url ~ '^gemini://[^/]+/?$';