refactor(logs): name every logger after its module, and hold it there

25 literal names covered 34 modules. plex/client.py, plex/stream.py,
plex/subtitles.py, plex/sync.py and two Plex route modules all called themselves
`siftlode.plex`; downloads/pipeline.py called itself `siftlode.worker`;
routes/channels.py called itself `siftlode.api`. The `logger` column is in every
stored row so E22.6's console can filter on it, and so an operator can pin a level
to one part of the system — neither works when six modules share a name.

`logs.get_logger(__name__)` derives it: app/plex/sync.py -> siftlode.plex.sync.
One name per module, and it cannot drift from the file it sits in.
tests/test_logger_names.py pins the derivation, that everything stays under the
one `siftlode` tree the handler and the tests attach to, and that no module
hand-rolls a name again unless the line says why (`# logger-name-ok: <why>`).
Two do: get_logger's own call, and `siftlode.http`, which is named for the traffic
it carries rather than the module it lives in — it replaced uvicorn's access log
and is what an operator pins when they want request lines and nothing else.

app/scheduler.py called its logger `logger`; every other module calls it `log`.

ruff gains LOG, G, T20, TRY400, TRY401 and S112. Four findings cleared with them:
worker.py logged `str(exc)[:120]` where the traceback was the whole question, and
three prints/f-strings that are deliberate now say so on the line. S110 and BLE001
stay off until S3 — 23 handlers are outstanding and each is a judgement about one
of them; enabling the rules first would just mean 23 blanket noqas.

backend/app/plex/stream.py

@@ -18,7 +18,6 @@ segments. The remux is CPU-light (video copy + a tiny audio transcode), so it ru
 CPU-only prod host; only P3's full transcode is CPU-heavy.
 """
 
-import logging
 import os
 import re
 import secrets
@@ -31,15 +30,14 @@ from pathlib import Path
 
 from sqlalchemy.orm import Session as DbSession
 
-from app import sysconfig
+from app import logs, sysconfig
 from app.config import settings
 from app.downloads.edit import encode_thread_cap
 from app.models import PlexItem
 from app.plex import paths
 from app.plex.sync import HW_DECODABLE_CODECS
 
-log = logging.getLogger("siftlode.plex")
-
+log = logs.get_logger(__name__)
 _HLS_ROOT = (
     Path(settings.plex_hls_dir)
     if settings.plex_hls_dir