Fixes "embedding request debug on radeon disappeared" — a real regression
where the radeon engine stopped emitting ALL log output the instant it
finished loading its GGUF embedders (~13s after launch), while still serving,
so its FULL REQUEST DEBUG + thermal debug vanished with no trace.

Two changes:

1) codai/api/log.py — the log_requests middleware now ALSO appends each
   FULL REQUEST DEBUG block, from the engine's own process, straight to
   <dir(CODERAI_LOG_FILE)>/requests.log via a single atomic O_APPEND write
   (tagged [ts][engine] from CODERAI_ENGINE_NAME). This survives the engine's
   stdout stream dying, so request bodies are captured regardless. Concurrent
   engines never interleave mid-block.

2) codai/frontproxy/engine_supervisor.py — _pump_logs no longer dies silently:
   a bad line can't kill the pump, and on stream end it prints
   "[pump] log stream ended (<stdout EOF|pump error>); engine still ALIVE/exited"
   so a vanishing engine log stream is diagnosable instead of looking like
   "debug turned off".
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R6iugoNeEgt9PkajKrFyq4