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>

.github/workflows/test.yml

@@ -35,7 +35,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -r requirements.txt -r requirements-test.txt
+          pip install -r requirements.txt -r web_interface/requirements.txt -r requirements-test.txt
           pip install RGBMatrixEmulator
 
       - name: Run plugin safety harness
@@ -58,7 +58,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -r requirements.txt -r requirements-test.txt
+          pip install -r requirements.txt -r web_interface/requirements.txt -r requirements-test.txt
           pip install RGBMatrixEmulator
 
       # Run the ENTIRE test tree (except test/plugins, which the