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

bandit.yaml

@@ -0,0 +1,33 @@
+# bandit.yaml — LEDMatrix bandit configuration
+# https://bandit.readthedocs.io/en/latest/config.html
+#
+# Skips are justified by the specific codebase context documented below.
+# Do not remove skips without updating the justification comment.
+
+skips:
+  # B104: Binding to all interfaces (0.0.0.0)
+  # Intentional — the Flask server binds 0.0.0.0 for LAN access on a Raspberry Pi.
+  # This is not internet-facing and is documented in web_interface/app.py.
+  - B104
+
+  # B603: subprocess call without shell=True
+  # All subprocess.run() calls in this codebase use list arguments (confirmed by
+  # grep — zero uses of shell=True in src/ or web_interface/). List args prevent
+  # shell injection. See src/common/permission_utils.py for the primary usage.
+  - B603
+
+  # B607: Starting a process with a partial executable path
+  # The subprocess calls invoke system utilities (systemctl, sudo, git) by name.
+  # These are fixed-list invocations, not user-controlled, and rely on PATH.
+  - B607
+
+exclude_dirs:
+  - tests
+  - test
+  - venv
+  - .venv
+  - rpi-rgb-led-matrix-master
+  # prove_security.py intentionally contains detection patterns as string literals
+  # (e.g. "eval(", "exec(") to search for in other files — bandit would flag
+  # these as false positives.
+  - scripts/prove_security.py