fix(axelera): accept scheme-less host URLs in model.path

is_url() required an http(s) scheme, so a bare host like
www.example.com/model.axm fell through to the filesystem branch and
raised FileNotFoundError, blocking detector init on a valid remote
model (reviewer P1). A value matching a leading dotted host (or
localhost) with a path is now recognised as a URL and http:// is
prepended at fetch time; relative paths and preset names behave as
before.

frigate/detectors/plugins/axelera.py

@@ -27,6 +27,7 @@ Model configuration follows the hardware-backend convention:
 import logging
 import os
 import queue
+import re
 import threading
 import time
 from typing import Literal
@@ -78,9 +79,31 @@ TOKEN_NO_ID = 0
 # ----------------- Utility Functions ----------------- #
 
 
+# Matches a scheme-less host URL: host([:port])/path, e.g. www.example.com/model.axm
+# or localhost:8000/model.axm (bare filenames like foo.axm and relative paths like
+# models/foo.axm have no leading dotted host and stay local paths / presets).
+_HOST_URL_RE = re.compile(
+    r"^(localhost|[a-zA-Z0-9-]+(\.[a-zA-Z0-9-]+)*\.[a-zA-Z]{2,})(:[0-9]+)?/"
+)
+
+
 def is_url(value: str) -> bool:
-    """Return True for well-formed http(s) URLs (scheme required)."""
-    return value.startswith(("http://", "https://"))
+    """Return True for http(s) URLs and scheme-less host URLs.
+
+    A bare host such as ``www.example.com/model.axm`` (no scheme) is a usable
+    URL too: rejecting it would raise FileNotFoundError on a valid remote
+    model. ``_url_with_scheme`` prepends ``http://`` before any fetch.
+    """
+    return value.startswith(("http://", "https://")) or bool(
+        _HOST_URL_RE.match(value)
+    )
+
+
+def _url_with_scheme(value: str) -> str:
+    """Prepend http:// to a scheme-less host URL (vetted by is_url)."""
+    if value.startswith(("http://", "https://")):
+        return value
+    return f"http://{value}"
 
 
 def _looks_like_path(value: str) -> bool:
@@ -105,7 +128,8 @@ def _resolve_network(model_path, model_url) -> str:
         if not os.path.isfile(destination):
             logger.info("axelera: downloading model from %s", model_url)
             try:
-                ModelDownloader.download_from_url(model_url, destination)
+                download_url = _url_with_scheme(model_url)
+                ModelDownloader.download_from_url(download_url, destination)
             except Exception as exc:
                 raise RuntimeError(
                     f"Failed to download model from {model_url}: {exc}"