Add GET /api/v1/probe endpoint for device inspection

Fast (~1-3s) endpoint that gathers network info about a device
before full stream discovery. Runs ping first, then parallel probes.

Features:
- Ping with ICMP + TCP fallback (works without root)
- Reverse DNS hostname lookup
- ARP table MAC address + OUI vendor identification (2403 entries, 51 camera vendors)
- mDNS HomeKit detection (camera/doorbell, paired status)
- Extensible Prober interface for adding new probe types
- 3-second overall timeout, parallel execution

Response includes "type" field:
- "unreachable" - device not responding
- "standard" - normal IP camera (RTSP/HTTP/ONVIF flow)
- "homekit" - Apple HomeKit camera (PIN pairing flow)

internal/models/probe.go

@@ -0,0 +1,45 @@
+package models
+
+// ProbeResponse represents the result of probing an IP address.
+// The Type field determines which UI flow the frontend should use:
+//   - "unreachable" -- device did not respond to ping
+//   - "standard"    -- normal IP camera (RTSP/HTTP/ONVIF)
+//   - "homekit"     -- Apple HomeKit camera (needs PIN pairing)
+type ProbeResponse struct {
+	IP        string       `json:"ip"`
+	Reachable bool         `json:"reachable"`
+	LatencyMs float64      `json:"latency_ms,omitempty"`
+	Type      string       `json:"type"`
+	Error     string       `json:"error,omitempty"`
+	Probes    ProbeResults `json:"probes"`
+}
+
+// ProbeResults contains results from all parallel probers.
+// Nil fields mean the prober did not find anything or timed out.
+type ProbeResults struct {
+	DNS  *DNSProbeResult  `json:"dns"`
+	ARP  *ARPProbeResult  `json:"arp"`
+	MDNS *MDNSProbeResult `json:"mdns"`
+}
+
+// DNSProbeResult contains reverse DNS lookup result.
+type DNSProbeResult struct {
+	Hostname string `json:"hostname"`
+}
+
+// ARPProbeResult contains ARP table lookup + OUI vendor identification.
+type ARPProbeResult struct {
+	MAC    string `json:"mac"`
+	Vendor string `json:"vendor"`
+}
+
+// MDNSProbeResult contains mDNS service discovery result (HomeKit).
+type MDNSProbeResult struct {
+	Name     string `json:"name"`
+	DeviceID string `json:"device_id"`
+	Model    string `json:"model"`
+	Category string `json:"category"` // "camera", "doorbell"
+	Paired   bool   `json:"paired"`
+	Port     int    `json:"port"`
+	Feature  string `json:"feature"`
+}