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/feedquery.py

@@ -438,7 +438,7 @@ def feed_summary(db: Session, user: User, **params) -> dict:
         # Drop this category's own selections so its chips keep independent counts; a conjunctive
         # topic-AND keeps them (each remaining chip narrows to channels that ALSO have the selected).
         exclude = None if tag_modes.get(category, "or") == "and" else category
-        for cid, ch_tags in channel_tags.items():
+        for ch_tags in channel_tags.values():
             if not passes(ch_tags, exclude):
                 continue
             for tid in ch_tags:
@@ -671,8 +671,8 @@ def decode_cursor(keys: list[dict], cursor: str) -> tuple[list, str, datetime |
         raw = payload["k"]
         if len(raw) != len(keys):
             raise ValueError("key arity mismatch")
-        vals = [_cursor_dt(v) if k["kind"] == "dt" else v for k, v in zip(keys, raw)]
-        for k, v in zip(keys, vals):
+        vals = [_cursor_dt(v) if k["kind"] == "dt" else v for k, v in zip(keys, raw, strict=True)]
+        for k, v in zip(keys, vals, strict=True):
             if v is None and not k["nullable"]:
                 raise ValueError("null value for a non-nullable key")
         window = payload.get("w")