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/plex/stream.py

@@ -245,6 +245,9 @@ def _probe_video_kind(src: Path) -> str:
             capture_output=True,
             text=True,
             timeout=10,
+            # A non-zero exit is a failed probe (CalledProcessError, a SubprocessError → the branch
+            # below), not an answer: whatever it printed before refusing is not the file's truth.
+            check=True,
         )
     except (subprocess.SubprocessError, OSError) as exc:
         # The safe kind is a real answer, but it is the answer for an 8-bit SDR source — a blind
@@ -502,6 +505,9 @@ def _probe_audio_langs(src: Path) -> list[str]:
             capture_output=True,
             text=True,
             timeout=10,
+            # A non-zero exit is a failed probe (CalledProcessError, a SubprocessError → the branch
+            # below), not an answer: whatever it printed before refusing is not the file's truth.
+            check=True,
         )
         return [ln.strip() for ln in out.stdout.splitlines()]
     except (subprocess.SubprocessError, OSError) as exc:
@@ -615,6 +621,9 @@ def _probe_first_pts(seg: Path) -> float | None:
             capture_output=True,
             text=True,
             timeout=10,
+            # A non-zero exit is a failed probe (CalledProcessError, a SubprocessError → the branch
+            # below), not an answer: whatever it printed before refusing is not the file's truth.
+            check=True,
         )
         for line in out.stdout.splitlines():
             tok = line.strip().strip(",")