chore: remove dead modules and unused dependencies (~1,180 LOC)

Deletions, each re-verified with a fresh repo-wide grep (core, web,
scripts, docs, plugin monorepo) immediately before removal:

Modules with zero live importers:
- src/background_cache_mixin.py + src/generic_cache_mixin.py (134+150
  LOC — referenced only by each other)
- src/font_test_manager.py (134 LOC)
- src/image_utils.py (22 LOC, self-documented deprecated)
- src/layout_manager.py (408 LOC — only its own test imported it) +
  test/test_layout_manager.py
- src/common/basketball_plugin_example.py (328 LOC sample)

requirements.txt entries with zero importers in core (pre-plugin-era
manager deps): icalevents, geopy, timezonefinder, unidecode. Plus the
google-auth trio (google-auth-oauthlib, google-auth-httplib2,
google-api-python-client): their only importer is the calendar PLUGIN,
which declares all three in its own requirements.txt (verified in the
monorepo and on an installed copy) — the plugin dependency installer
owns them. Existing venvs are unaffected (removal doesn't uninstall);
fresh installs get them when calendar is installed.

Two stale references cleaned (a comment in test_pillow_compat.py, a
directory listing in HOW_TO_RUN_TESTS.md). Full suite green except the
two documented pre-existing failures (circuit_breaker mock drift, fixed
in #400; clock-simple 64x32 overflow, pre-dates this series); all core
entry modules verified importing cleanly under the emulator.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FqzC1nzTWL4kaqgMaQZFam

.github/workflows/tests.yml

@@ -0,0 +1,46 @@
+name: Tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: pytest (Python ${{ matrix.python-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ['3.10', '3.11', '3.12']
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+        with:
+          submodules: false  # rgbmatrix submodule not needed in EMULATOR mode
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+          cache: pip
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+          # Optional deps that some test modules import
+          pip install scipy psutil Flask-Limiter
+
+      - name: Run tests
+        env:
+          EMULATOR: "true"
+        run: |
+          pytest \
+            -m "not hardware and not slow" \
+            --tb=short