add Prometheus metrics endpoint and push script

New /metrics endpoint exposes WiFi signal, bitrate, gateway,
system uptime/load/memory, and ping reachability in Prometheus
exposition format. skyweave-push.sh loops every 30s pushing
metrics to the pushgateway on deltrian.

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

cgi_scripts/skyweave.lua

@@ -45,6 +45,7 @@ local success, load_result = pcall(function()
     local network_handlers = require("skyweave.handlers.network")
     local system_handlers = require("skyweave.handlers.system")
     local wifi_handlers = require("skyweave.handlers.wifi")
+    local metrics_handlers = require("skyweave.handlers.metrics")
 
     local router = Router:new()
 
@@ -54,6 +55,7 @@ local success, load_result = pcall(function()
     router:add_route("GET", "/SOS", network_handlers.set_connect_wifi_sos)
     router:add_route("GET", "/wifi_scan", wifi_handlers.get_wifi_scan)
     router:add_route("GET", "/system_log_parsed", system_handlers.get_system_log_parsed)
+    router:add_route("GET", "/metrics", metrics_handlers.get_metrics)
 
     local request = request_context_builder.create_request_context()