stream.py enforced a hardcoded _MAX_SESSIONS = 4 and never read the
DB-overridable plex_max_transcodes ConfigSpec, so the Configuration → Plex →
"Max concurrent transcodes" knob was dead (same env-vs-DB drift class as the
Downloads/Admin fixes). _enforce_cap(cap) now takes the cap from
sysconfig.get_int(db, "plex_max_transcodes") (>=1 so playback can't be capped to
zero). Bumped the config default 1→4 so the effective default matches the old
hardcoded cap (no behavior change for non-overriders; the knob now works).