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>

db.py

@@ -178,6 +178,36 @@ def upsert_link(chassis_a, port_a, chassis_b, port_b):
     conn.close()
 
 
+def prune_stale_links(chassis_id, current):
+    """
+    Drop links on a just-scanned switch that its own LLDP table no longer backs.
+    current = set of (local_port, neighbor_chassis_id) the switch reported this
+    scan. Dept scans don't clear_links() first, so without this a re-cabled port
+    (e.g. a new switch inserted mid-run) keeps its old link alongside the new one.
+    Must run after the switch's upsert_link calls, which rewrite its side of each
+    surviving link to the self-reported port name compared here.
+    """
+    conn = get_conn()
+    rows = conn.execute("""
+        SELECT id, chassis_a, port_a, chassis_b, port_b FROM links
+        WHERE chassis_a=? OR chassis_b=?
+    """, (chassis_id, chassis_id)).fetchall()
+    stale = []
+    for r in rows:
+        if r['chassis_a'] == chassis_id:
+            key = (r['port_a'], r['chassis_b'])
+        else:
+            key = (r['port_b'], r['chassis_a'])
+        if key not in current:
+            stale.append((r['id'],))
+    conn.executemany("DELETE FROM links WHERE id=?", stale)
+    conn.commit()
+    conn.close()
+    if stale:
+        logger.info(f"prune_stale_links: removed {len(stale)} stale link(s) on {chassis_id}")
+    return len(stale)
+
+
 def clear_stale_switches(since_dt):
     """Remove switches not seen since the given datetime string."""
     conn = get_conn()