fix: one retry layer for background fetches; CI installs the web requirements; one Discord invite (#657)

- BackgroundDataService: the session adapter retried connection errors 3x
  inside each attempt of the service's own retry loop (up to 16 connection
  attempts per request on a dead network). The adapter no longer retries;
  ESPN date chunks, which bypass the loop and skip a failed chunk, get a
  small connection retry of their own (_ConnectionRetryingSession).
- CI installs web_interface/requirements.txt. The brotli header test now
  checks its intent (core never hand-sets br; requests may advertise it when
  a decoder is installed) instead of failing whenever brotli is present.
- Every Discord link uses the LEDMatrix server's invite (RdrC37rEag).

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

README.md

@@ -33,7 +33,7 @@ I'm trying to be open to constructive criticism and support, as long as it's a r
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