ppf: add periodic re-seeding of proxy source URLs

Seed sources that error out are permanently excluded from claiming.
Over time this starves the pipeline. Re-seed every 6 hours with
error reset for exhausted sources, preventing the starvation loop
that caused the previous outage.

ppf.py

@@ -1045,8 +1045,15 @@ def main():
     statusmsg = time.time()
     list_max_age_seconds = config.ppf.list_max_age_days * 86400
     last_skip_log = 0
+    last_reseed = time.time()
+    reseed_interval = 6 * 3600  # re-seed sources every 6 hours
     while True:
         try:
+            # Periodic re-seeding: reset errored-out seed sources
+            if time.time() - last_reseed >= reseed_interval:
+                dbs.seed_proxy_sources(urldb, reset_errors=True)
+                last_reseed = time.time()
+
             # When ppf threads = 0, skip URL fetching (workers handle it via /api/claim-urls)
             if config.ppf.threads == 0:
                 time.sleep(60)