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/routes/feed.py

@@ -133,7 +133,7 @@ def get_feed(
     # Expose each key column on the row so we can build the next cursor from the last item.
     query = query.add_columns(*[k["expr"].label(f"_k{i}") for i, k in enumerate(keys)])
     if vals is not None:
-        for k, v in zip(keys, vals):
+        for k, v in zip(keys, vals, strict=True):
             k["value"] = v
     if ranked and window is not None:
         rows = windowed_relevance_rows(db, query, keys, window, cursor_id, limit + 1)
@@ -431,7 +431,7 @@ def get_video_detail(
         ):
             items = yt.get_videos([video_id])
     except YouTubeError as exc:
-        raise youtube_refusal(exc)
+        raise youtube_refusal(exc) from exc
     if not items:
         raise not_found("video")