fix(plex): make the HW->SW fallback supersede-check atomic; honest cull yield

Deep-review round 2 on the round-1 fix:

- Race (medium): the round-1 identity guard did the supersede check and the
  respawn in TWO separate _lock sections. A newer same-key session installed in
  the gap was still torn down by _spawn's unconditional pop+kill (the round-1
  comment even had the direction backwards). Fold the check into _spawn via a
  new replace_only= param: the identity test and the pop+replace now share ONE
  lock, so the respawn is a true no-op when superseded/culled.

- Honest states (low): the superseded branch returned current_session(key),
  which is None when the key was cap-culled during our out-of-lock wait; the
  route turns None into a dishonest 'Media file not found on disk' 404 for a
  file that exists. Yield 'current_session(key) or s' so a culled session
  degrades to the route's retryable 'not available yet' path instead.

- Hygiene: _want_hw docstring corrected (it is a test-only shim now; production
  decides via _select_encode) and the two stale _want_hw references refreshed
  (stream.py header, config.py plex_transcode_hw).

- tests: the cap-cull no-respawn/no-None sub-case; the _opencl_present except
  OSError branch (glob raising, not just empty); tightened a tautological -vf
  assertion. Backend gate green (ruff + full pytest).

backend/app/config.py

@@ -265,7 +265,7 @@ class Settings(BaseSettings):
     # --- Transcode (playable="transcode") re-encode tuning (E6 S1) ---
     # Hardware-encoder policy: "auto" = VAAPI (h264_vaapi) when the iGPU render node is present AND the
     # source codec is hardware-decodable, else libx264 software; "on" = force VAAPI wherever the render
-    # node exists (skip the decodability gate); "off" = always software. See plex/stream.py `_want_hw`.
+    # node exists (skip the decodability gate); "off" = always software. See plex/stream.py `_select_encode`.
     plex_transcode_hw: str = "auto"
     # Software-fallback libx264 tuning (IGNORED on the VAAPI path). `preset` trades CPU for size; `crf`
     # trades quality for size (lower = better + bigger). `max_height` caps the SOFTWARE transcode's output