Add single-switch scan

New "Scan Switch" header button reuses the Test Connection picker and
posts to /api/scan/switch, which runs the normal scan pipeline on one
switch from the NocoDB inventory. Like a dept scan it does not clear
links first, so the rest of the map is left in place.

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

app.py

@@ -37,11 +37,12 @@ def _reschedule(interval_minutes):
     logger.info(f"Auto-scan scheduled every {interval_minutes} minutes")
 
 
-def _trigger_scan_background(dept: str = None, workers: int = 5, login_delay: int = 3):
+def _trigger_scan_background(dept: str = None, workers: int = 5, login_delay: int = 3, only_ip: str = None):
     """Run scan in a background thread. dept=None → all switches."""
     if not scanner.scan_state["running"]:
         t = threading.Thread(target=scanner.run_scan,
-                             kwargs={"dept": dept, "workers": workers, "login_delay": login_delay},
+                             kwargs={"dept": dept, "workers": workers, "login_delay": login_delay,
+                                     "only_ip": only_ip},
                              daemon=True)
         t.start()
 
@@ -132,6 +133,25 @@ def api_scan_ami():
     return jsonify({"status": "started", "dept": "AMI"})
 
 
+@app.route("/api/scan/switch", methods=["POST"])
+def api_scan_switch():
+    """Scan a single switch (by IP, must be in the NocoDB inventory)."""
+    if scanner.scan_state["running"]:
+        return jsonify({"error": "Scan already running"}), 409
+    ip = ((request.get_json(silent=True) or {}).get("ip") or "").strip()
+    if not ip:
+        return jsonify({"error": "ip is required"}), 400
+    try:
+        known = any(s["ip"] == ip for s in get_switches())
+    except RuntimeError as e:
+        return jsonify({"error": str(e)}), 502
+    if not known:
+        return jsonify({"error": f"{ip} is not an active switch in NocoDB"}), 404
+    workers, login_delay = _scan_params()
+    _trigger_scan_background(workers=workers, login_delay=login_delay, only_ip=ip)
+    return jsonify({"status": "started", "ip": ip})
+
+
 @app.route("/api/scan/abort", methods=["POST"])
 def api_scan_abort():
     """Request the in-progress scan to stop after in-flight switches finish."""