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/app/issues/gitea.py

@@ -239,7 +239,7 @@ def deliver(
                 # already open on the tracker and this is what would have recorded its link. We
                 # still must not write — another send owns the row and is opening its own issue —
                 # so say which issue was left behind, because nothing else will.
-                # noqa TRY400: the traceback is one SendClaimLost carrying the session id and
+                # TRY400: the traceback is one SendClaimLost carrying the session id and
                 # nothing else; the issue number below is the whole news.
                 log.error(  # noqa: TRY400
                     "an issue was created by a send that had already lost its claim",