Serve per-client clash.yaml dynamically via mtr-backend

- clash.yaml template: proxy-provider URL uses ${SUB_ID} placeholder
- install_clash_sub() saves to clash.yaml.tpl (DOMAIN/SUB_PATH substituted, SUB_ID left)
- mtr-backend GET /api/clash?sub_id=xxx reads .tpl, fills in subscription ID, returns YAML
- nginx regex ^/sub_path/(?<clash_sub_id>[^/]*)$ proxies Clash UA to
  mtr-backend /api/clash?sub_id=$clash_sub_id; regular clients pass through to x-ui
- Removed static /__clash_sub internal location

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

assets/clash/clash.yaml

@@ -19,7 +19,7 @@ lgbm-url: "https://github.com/vernesong/mihomo/releases/download/LightGBM-Model/
 proxy-providers:
   sub:
     type: http
-    url: https://${DOMAIN}/${SUB_PATH}/
+    url: https://${DOMAIN}/${SUB_PATH}/${SUB_ID}
     path: ./proxy_providers/base64.yml
     interval: 3600
     health-check:

assets/diagnostics/mtr-backend.py

@@ -180,10 +180,38 @@ class Handler(BaseHTTPRequestHandler):
         return ip
 
     def do_GET(self):
-        if self.path == "/health":
+        parsed = urlparse(self.path)
+
+        if parsed.path == "/health":
             self._send_json(200, {"ok": True})
-        else:
-            self._send_json(404, {"error": "not found"})
+            return
+
+        # ── Clash subscription generator ───────────────────────────────────────
+        if parsed.path == "/api/clash" or parsed.path.endswith("/api/clash"):
+            tpl_path = "/var/www/subpage/clash.yaml.tpl"
+            if not os.path.isfile(tpl_path):
+                self._send_json(404, {"error": "clash template not found"})
+                return
+            params = parse_qs(parsed.query)
+            sub_id = params.get("sub_id", [""])[0].strip()
+            try:
+                with open(tpl_path, "r", encoding="utf-8") as f:
+                    content = f.read()
+                content = content.replace("${SUB_ID}", sub_id)
+                payload = content.encode("utf-8")
+                self.send_response(200)
+                self.send_header("Content-Type", "text/yaml; charset=utf-8")
+                self.send_header("Content-Length", str(len(payload)))
+                self.send_header("Content-Disposition", "attachment; filename=clash.yaml")
+                self.send_header("Cache-Control", "no-store")
+                self.end_headers()
+                self.wfile.write(payload)
+            except Exception as exc:
+                log.error("clash template error: %s", exc)
+                self._send_json(500, {"error": "internal error"})
+            return
+
+        self._send_json(404, {"error": "not found"})
 
     def do_POST(self):
         parsed = urlparse(self.path)