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/plex/collections.py

@@ -428,7 +428,7 @@ def create_collection(
         db.commit()
     except (PlexError, PlexNotConfigured) as e:
         db.rollback()
-        raise _write_or_pending_error(wrote, e)
+        raise _write_or_pending_error(wrote, e) from e
     return _collection_card(col)
 
 
@@ -450,7 +450,7 @@ def collection_add_item(
         db.commit()
     except (PlexError, PlexNotConfigured) as e:
         db.rollback()
-        raise _write_or_pending_error(wrote, e)
+        raise _write_or_pending_error(wrote, e) from e
     return _collection_card(col)
 
 
@@ -472,7 +472,7 @@ def collection_remove_item(
         db.commit()
     except (PlexError, PlexNotConfigured) as e:
         db.rollback()
-        raise _write_or_pending_error(wrote, e)
+        raise _write_or_pending_error(wrote, e) from e
     return _collection_card(col)
 
 
@@ -502,7 +502,7 @@ def collection_rename(
         db.commit()
     except (PlexError, PlexNotConfigured) as e:
         db.rollback()
-        raise _write_or_pending_error(wrote, e)
+        raise _write_or_pending_error(wrote, e) from e
     return _collection_card(col)
 
 

backend/app/routes/plex/discover.py

@@ -124,8 +124,14 @@ def _recently_added(
     # share one), so the merge doesn't depend on object identity staying alive.
     top_movies = [o for _, kind, o in top if kind == "movie"]
     top_shows = [o for _, kind, o in top if kind == "show"]
-    mcards = {m.rating_key: c for m, c in zip(top_movies, _movie_cards(db, uid, top_movies, lang))}
-    scards = {s.rating_key: c for s, c in zip(top_shows, _show_cards(db, uid, top_shows, lang))}
+    mcards = {
+        m.rating_key: c
+        for m, c in zip(top_movies, _movie_cards(db, uid, top_movies, lang), strict=True)
+    }
+    scards = {
+        s.rating_key: c
+        for s, c in zip(top_shows, _show_cards(db, uid, top_shows, lang), strict=True)
+    }
     return [mcards[o.rating_key] if kind == "movie" else scards[o.rating_key] for _, kind, o in top]
 
 
@@ -351,10 +357,12 @@ def _because_you_watched(
     ordered_shows = [shows[rk] for rk in rks if rk in shows]
     # rating_key is globally unique in Plex, so it keys the serialized cards back safely (no id()).
     mcards = {
-        m.rating_key: c for m, c in zip(ordered_movies, _movie_cards(db, uid, ordered_movies, lang))
+        m.rating_key: c
+        for m, c in zip(ordered_movies, _movie_cards(db, uid, ordered_movies, lang), strict=True)
     }
     scards = {
-        s.rating_key: c for s, c in zip(ordered_shows, _show_cards(db, uid, ordered_shows, lang))
+        s.rating_key: c
+        for s, c in zip(ordered_shows, _show_cards(db, uid, ordered_shows, lang), strict=True)
     }
     items: list[dict] = []
     for rk in rks: