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>

src/background_data_service.py

@@ -170,14 +170,10 @@ class BackgroundDataService:
         self.session.mount('http://', requests.adapters.HTTPAdapter(max_retries=3))
         self.session.mount('https://', requests.adapters.HTTPAdapter(max_retries=3))
         
-        # Default headers
-        self.default_headers = {
-            'User-Agent': 'LEDMatrix/1.0 (https://github.com/yourusername/LEDMatrix)',
-            'Accept': 'application/json',
-            'Accept-Language': 'en-US,en;q=0.9',
-            'Accept-Encoding': 'gzip, deflate, br',
-            'Connection': 'keep-alive'
-        }
+        # Default headers: core's shared set (real User-Agent, no hand-set
+        # Accept-Encoding) -- see src/common/api_helper.py.
+        from src.common.api_helper import DEFAULT_HTTP_HEADERS
+        self.default_headers = dict(DEFAULT_HTTP_HEADERS)
         
         logger.info(f"BackgroundDataService initialized with {max_workers} workers")