Prune links a scanned switch no longer reports

Dept scans don't clear links first, so when a port was re-cabled (e.g. a
new switch inserted on an existing uplink) the old link stayed alongside
the new one. After storing a switch's neighbors, prune_stale_links() now
deletes any link on that switch that its own LLDP table doesn't back.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scanner.py

@@ -4,7 +4,7 @@ import threading
 from nocodb_client import get_switches
 from ssh_client import scan_all_switches
 from db import (
-    upsert_switch, upsert_link, clear_links,
+    upsert_switch, upsert_link, clear_links, prune_stale_links,
     log_scan_start, log_scan_finish, merge_duplicate_switches
 )
 from parser import parse_neighbor_description
@@ -126,6 +126,10 @@ def run_scan(dept: str = None, workers: int = 5, login_delay: int = 3):
                     chassis_b=neighbor["chassis_id"],
                     port_b=neighbor["remote_port"],
                 )
+            prune_stale_links(
+                result["chassis_id"],
+                {(n["local_port"], n["chassis_id"]) for n in result["neighbors"]},
+            )
         else:
             scan_state["fail"] += 1
             error = result.get("error", "Unknown error")