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/.basedpyright/baseline.json

@@ -1650,24 +1650,6 @@
                     "endColumn": 36,
                     "lineCount": 1
                 }
-            },
-            {
-                "code": "reportArgumentType",
-                "range": {
-                    "startColumn": 46,
-                    "endColumn": 52,
-                    "lineCount": 1
-                }
-            }
-        ],
-        "./app/youtube/rss.py": [
-            {
-                "code": "reportGeneralTypeIssues",
-                "range": {
-                    "startColumn": 10,
-                    "endColumn": 15,
-                    "lineCount": 1
-                }
             }
         ]
     }