fix(scanner): decouple port discovery from version detection

nmap -sV sends plaintext probes to TLS ports (e.g. Proxmox 8006). They
stall until the host crosses --host-timeout, at which point nmap skips
the whole host and discards ports it already found — a reachable host is
returned with 0 open ports (issue #277).

Split Phase 2 into two passes: Pass A (-sS/-sT --open) discovers ports
fast with no version detection; its ports are authoritative. Pass B
(-sV --host-timeout, scoped to the found ports) enriches banners
best-effort — on timeout/failure the discovered ports are kept with
empty banners instead of losing the host.

Healthy hosts still get full-intensity -sV banners (no quality drop).
Version host-timeout is now configurable via SCANNER_VERSION_HOST_TIMEOUT
(default 60s).

ha-relevant: yes

backend/app/core/config.py

@@ -52,6 +52,11 @@ class Settings(BaseSettings):
     # Scanner
     scanner_ranges: list[str] = ["192.168.1.0/24"]
 
+    # Phase-2 version-detection (-sV) host timeout, seconds. Bounds the version
+    # pass so a stalling TLS port (e.g. Proxmox 8006) can't hang it. Discovered
+    # ports survive a timeout regardless; raise this on slow/overlay networks.
+    scanner_version_host_timeout: int = 60
+
     # Deep scan — persisted defaults (overridable per-scan from the scan dialog).
     # http_ranges: extra nmap port ranges, opt-in, no default. Probe + TLS off by default.
     scanner_http_ranges: list[str] = []