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/main.py

@@ -77,7 +77,7 @@ async def lifespan(app: FastAPI):
         # booting — an unreachable or not-yet-migrated database has to start into the env level.
         try:
             set_level(sysconfig.get_str(db, "log_level"))
-        except Exception:  # noqa: BLE001 — any DB/config failure keeps the bootstrap level
+        except Exception:  # any DB/config failure keeps the bootstrap level
             # With the traceback: this catch spans a connection, a query and a coercion at boot,
             # and `%s` of the exception does not say which of them raised.
             log.exception("could not read the stored log level; staying at %s", current_level())