feat: add soft scan mode (slow ICMP) to avoid switch/AP rate-limiting

Reduces ICMP concurrency from 100 to 10 workers when soft_scan=true,
spreading out probes to avoid rate-limiting on managed switches and APs.
The option is hidden in the UI when TCP check is active (redundant).

Update README (en/fr/es), docs/backend.md with the new scan modes table
and a troubleshooting entry for ICMP rate-limiting.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/discovery.py

@@ -29,6 +29,7 @@ class ScanRequest(BaseModel):
     dns_server: str = ""
     targets: list[ScanTarget]
     tcp_check: bool = False
+    soft_scan: bool = False
 
     @field_validator("dns_server")
     @classmethod
@@ -189,7 +190,11 @@ def scan(req: ScanRequest):
     t0 = time.time()
     results: list[DiscoveredHost] = []
 
-    with ThreadPoolExecutor(max_workers=100) as pool:
+    # Soft scan reduces ICMP concurrency to avoid rate-limiting on switches/APs.
+    # Has no effect in tcp_check mode (TCP probes are not rate-limited the same way).
+    workers = 10 if (req.soft_scan and not req.tcp_check) else 100
+
+    with ThreadPoolExecutor(max_workers=workers) as pool:
         futures = [pool.submit(_scan_one, *args, tcp_check=req.tcp_check) for args in tasks]
         for f in as_completed(futures):
             host = f.result()