fix(stock-webai): final review notes — env default + 1-time auth error log

(1) docker-compose: ${WEBAI_API_KEY} → ${WEBAI_API_KEY:-} matches
project convention, avoids "variable not set" warning when NAS .env
lacks the key during initial deploy.

(2) auth.py: ERROR log when WEBAI_API_KEY env unset fires only on
first miss, then silent (module-level _WEBAI_AUTH_WARNED flag).
Flag resets when env becomes configured, so future regressions log
again. Eliminates log spam under web-ai polling (~3/min).

All 102 tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -43,7 +43,7 @@ services:
       - OLLAMA_URL=${OLLAMA_URL:-http://192.168.45.59:11435}
       - OLLAMA_MODEL=${OLLAMA_MODEL:-qwen3:14b}
       - CORS_ALLOW_ORIGINS=${CORS_ALLOW_ORIGINS:-http://localhost:3007,http://localhost:8080}
-      - WEBAI_API_KEY=${WEBAI_API_KEY}
+      - WEBAI_API_KEY=${WEBAI_API_KEY:-}
     volumes:
       - ${RUNTIME_PATH}/data/stock:/app/data
     healthcheck: