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/pytest.ini

@@ -2,7 +2,9 @@
 # Run from backend/ (the image's /app). `pythonpath = .` puts the app package on sys.path so tests
 # import it as `app.*`, matching runtime. Most of the suite is pure-logic; the DB-backed lane opts in
 # via the `db` fixture and runs in the compose test image. `-rs` reports skip REASONS so a silently
-# skipped lane is visible (the compose lane sets SIFTLODE_TEST_REQUIRE_DB=1 to make skips a hard error).
+# skipped lane is visible (and conftest makes a missing DB/ffmpeg a hard error unless
+# SIFTLODE_TEST_REQUIRE_DB=0). `--strict-markers` (E32): a misspelt `@pytest.mark.skipiff` is an error,
+# not an unknown marker that quietly does nothing.
 testpaths = tests
 pythonpath = .
-addopts = -q -rs
+addopts = -q -rs --strict-markers