hotfix(v1.7.9): auth diagnostics and rate-limit exemptions

- Exempt /api/config/auth, /api/config/features, /health, /metrics from rate limiting
- Fix generic exception handler to return proper JSON for HTTPException instead of re-raising
- Add startup log with auth_enabled and version
- Add frontend console logging for auth config fetch errors
- Show 'Auth: OFF' or 'Auth: misconfigured' on auth button instead of empty text
- Add backend debug logging to /api/config/auth endpoint

backend/routes/config.py

@@ -1,3 +1,4 @@
+import structlog
 from config import (
     AI_FEATURES_ENABLED,
     AUTH_CLIENT_ID,
@@ -9,10 +10,12 @@ from config import (
 from fastapi import APIRouter
 
 router = APIRouter()
+logger = structlog.get_logger("aoc.config")
 
 
 @router.get("/config/auth")
 def auth_config():
+    logger.debug("Auth config requested", auth_enabled=AUTH_ENABLED)
     return {
         "auth_enabled": AUTH_ENABLED,
         "tenant_id": AUTH_TENANT_ID,