fix(test): stop sys.modules-deleting backend.app.main in test_code_quality

+ ci: shard backend tests 4-way + drop -v for ~3.5x wall-clock speedup

  Root cause of the 4 CI failures on PR #1514 (all in
  test_print_start_assigns_printer_id_to_vp_archive.py +
  test_timelapse_baseline_restart_recovery.py): test_all_modules_importable
  in test_code_quality.py was deleting backend.app.main from sys.modules
  and re-importing it via importlib.import_module. That created NEW
  module-level dicts (_timelapse_baselines, _expected_prints,
  _active_prints, …) and re-ran root_logger.addHandler — hence the
  duplicate log lines at the same microsecond in captured stderr.

  Any sibling test that bound those names via "from backend.app.main
  import _timelapse_baselines" before the reimport now held a reference
  to the OLD dict; production code (reached via "from backend.app.main
  import on_print_start") resolved the symbol through the NEW module
  instance. Production mutated the new dict, the test read the old one,
  the assertion saw None / un-mutated mock_archive.

  Locally with -n 30, xdist load-balanced test_code_quality.py to a
  different worker process so the collision never happened (which is
  why the suite was green for me). CI's -n auto = -n 2 on ubuntu-latest
  made the collision deterministic.

  Fix: drop the "del sys.modules[name]" step. importlib.import_module
  already returns the cached module if cached, or runs the import
  machinery if not — either way, any import-time error surfaces. The
  "fresh import" framing was theatre; in practice every module in the
  list is already imported by other tests/fixtures before this test
  runs, so we were never actually getting a fresh import anyway — just
  destruction.

  CI workflow tightening (separate concern, same PR since both touch
  the test infrastructure):

  - Dropped -v from the pytest invocation. 5300+ "PASSED foo::bar"
    lines per worker were eating ~30-60s of stdout I/O on 2-vCPU
    runners. --tb=short is sufficient for failure context.
  - Sharded backend-tests into a 4-way matrix via pytest-split (new
    dev dep). Each shard runs ~1326 tests in ~95s on a 2-vCPU runner;
    all 4 run in parallel so wall-clock drops from 362s -> ~100s.
  - fail-fast: false on the matrix so a single failing shard doesn't
    hide failures in the other three — PRs see the complete failure
    picture in one push.

backend/tests/unit/test_code_quality.py

@@ -218,9 +218,27 @@ class TestModuleImports:
         """Verify all Python modules can be imported without errors.
 
         This catches syntax errors and missing dependencies.
+
+        IMPORTANT: We must NOT ``del sys.modules[name]`` to force a fresh
+        import here. ``backend.app.main`` is a stateful module — re-importing
+        it builds NEW module-level dicts (_timelapse_baselines,
+        _expected_prints, _active_prints, …) and re-runs ``root_logger.
+        addHandler(console_handler)``. Any test that already bound those
+        names via ``from backend.app.main import _timelapse_baselines`` now
+        holds a stale reference, while production code resolves the symbol
+        through the new module instance — they're two different dicts. CI
+        under -n 2 puts test_code_quality.py on the same worker as
+        test_print_start_assigns_printer_id_to_vp_archive.py and
+        test_timelapse_baseline_restart_recovery.py, and those tests see
+        their mock_archive un-mutated / their baseline dict empty even
+        though production logged the mutations went through. Local -n 30
+        spreads the tests across workers and the collision never happens.
+
+        ``importlib.import_module`` already covers the "is this importable"
+        check — it returns the cached module if cached, or runs the import
+        machinery if not. Either way, an import-time error surfaces here.
         """
         import importlib
-        import sys
 
         # Modules to test importing
         modules = [
@@ -235,9 +253,6 @@ class TestModuleImports:
         errors = []
         for module_name in modules:
             try:
-                # Remove from cache first to ensure fresh import
-                if module_name in sys.modules:
-                    del sys.modules[module_name]
                 importlib.import_module(module_name)
             except Exception as e:
                 errors.append(f"{module_name}: {type(e).__name__}: {e}")