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/sync/runner.py

@@ -267,7 +267,7 @@ def run_feed_fallback(db: Session, channel_ids: list[str]) -> dict[str, int]:
                 channel.feed_fallback_at = datetime.now(timezone.utc)
                 db.add(channel)
                 db.commit()
-            except Exception:  # noqa: BLE001 — one channel must not stop the rest
+            except Exception:  # one channel must not stop the rest
                 db.rollback()
                 log.exception(
                     "Feed fallback could not store channel %s",