build(backend): close the lint gaps the E32 audit measured

ruff gains RUF100, PLW1510 and flake8-bugbear, every hit fixed rather than
ignored. B008 stays on with FastAPI's Depends/Query declared immutable.
basedpyright moves to standard mode, pytest to --strict-markers, and a hand
run of pytest now requires the database unless SIFTLODE_TEST_REQUIRE_DB=0.

PLW1510 found a real bug: the three ffprobe probes in plex/stream.py never
looked at the exit status, so a probe that failed was read as the file's
truth (an HDR source sent down the HW-safe path, a multi-audio file collapsed
to one track, a subtitle offset taken from garbage) and the warning each
except branch promises never fired. They now run with check=True.

backend/tests/test_logsink.py

@@ -259,7 +259,7 @@ def test_the_writer_drains_what_was_queued_when_it_is_stopped(db):
     log.addHandler(sink)
     try:
         for i in range(logsink.BATCH_MAX + 5):
-            # noqa G004: the f-string is the POINT — a distinct rendered message per line is
+            # G004: the f-string is the POINT — a distinct rendered message per line is
             # what keeps the flood limiter (which groups by template) out of this case.
             log.warning(f"line {i}")  # noqa: G004
     finally: