fix(logos): one hardened logo download path; shared, real HTTP headers (#612)

* fix(logos): harden the plugin logo download and share core HTTP headers

download_missing_logo / LogoDownloader.download_logo, the path the
scoreboard plugins use, read response.content with no size cap and wrote
straight to the final path, so a failed or corrupt download could be left
in place and cached as the logo. It now goes through fetch_logo: streamed
with a 10 MB cap, image/* only, decoded by Pillow, converted to RGBA once,
and moved into place atomically. A failure leaves no partial or temp file
and keeps any logo already on disk. LogoHelper._download_logo delegates to
the same code. Public signatures and return values are unchanged; saved
files are pixel-identical to before (RGBA, palette+tRNS, L+tRNS, LA, JPEG).

download_missing_logo reuses one downloader per thread instead of a new
Session per logo. Per thread rather than behind a lock: Session is not
documented thread-safe, and a lock would serialise every plugin's
downloads behind the slowest one.

Placeholders are written atomically, without the test_write.tmp probe.

The logo downloader and background data service now send the real
ChuckBuilds User-Agent from src.common.api_helper (USER_AGENT,
DEFAULT_HTTP_HEADERS) instead of a yourusername/contact@example.com
placeholder, and no longer hand-set Accept-Encoding: br (brotli is not
installed).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(http): drop APIHelper's hand-set brotli encoding; LogoHelper sends the real UA

APIHelper advertised `br` though brotli isn't installed, so a server that
honoured it would send a body requests can't decode. LogoHelper sent a bare
`LEDMatrix-Common/1.0`, the kind of User-Agent ESPN has been rejecting.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/common/api_helper.py

@@ -8,14 +8,32 @@ Extracted from LEDMatrix core to provide reusable functionality for plugins.
 import logging
 import time
 from datetime import datetime
+from types import MappingProxyType
 from src.common.espn_dates import ESPN_MAX_LIMIT
-from typing import Any, Dict, Optional
+from typing import Any, Dict, Mapping, Optional
 
 import requests
 from requests.adapters import HTTPAdapter
 from urllib3.util.retry import Retry
 
 
+#: The User-Agent core sends to ESPN and other data APIs. It names the client
+#: and links to it: around 2026-08-04 ESPN began 403ing bare custom tokens
+#: (and browser strings), and this form is what it accepts.
+USER_AGENT = 'LEDMatrix/1.0 (+https://github.com/ChuckBuilds/LEDMatrix)'
+
+#: Base headers for core's JSON API requests. Read-only; pass
+#: ``{**DEFAULT_HTTP_HEADERS, ...}`` to add to it. There is deliberately no
+#: Accept-Encoding: requests advertises only what urllib3 can decode here
+#: (``br`` needs the optional brotli package, which is not a requirement), so a
+#: hand-set ``br`` invites a body the client cannot read.
+DEFAULT_HTTP_HEADERS: Mapping[str, str] = MappingProxyType({
+    'User-Agent': USER_AGENT,
+    'Accept': 'application/json',
+    'Accept-Language': 'en-US,en;q=0.9',
+})
+
+
 class APIHelper:
     """
     Helper class for HTTP requests, caching, and ESPN API integration.
@@ -57,12 +75,9 @@ class APIHelper:
         
         # Default headers
         self.session.headers.update({
-            # Identifies the client and links to it: ESPN began 403ing bare
-            # custom tokens (and browser strings) around 2026-08-04.
-            'User-Agent': 'LEDMatrix/1.0 (+https://github.com/ChuckBuilds/LEDMatrix)',
+            'User-Agent': USER_AGENT,
             'Accept': 'application/json',
             'Accept-Language': 'en-US,en;q=0.9',
-            'Accept-Encoding': 'gzip, deflate, br',
             'Connection': 'keep-alive'
         })