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_body_limit.py

@@ -321,7 +321,7 @@ def _body_reading_functions() -> dict[tuple[str, str], dict]:
                 continue
             names = [name for _, name in calls]
             limits = []
-            for node, name in zip([n for n, _ in calls], names):
+            for node, name in zip([n for n, _ in calls], names, strict=True):
                 if name.startswith("request."):
                     limits.append("<no ceiling declared>")
                     continue