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/bodylimit.py

@@ -219,7 +219,7 @@ class BodyLimitMiddleware:
 
         try:
             await self.app(scope, counting_receive, guarded_send)
-        except Exception:  # noqa: BLE001 — only swallowed when WE caused it, see below
+        except Exception:  # only swallowed when WE caused it, see below
             # ClientDisconnect (or whatever a handler raises on a truncated read) is the expected
             # consequence of the disconnect we just faked, and the caller must get the 413 rather
             # than a 500. Anything raised when we did NOT refuse is a real failure and is re-raised