security: fix critical vulnerabilities (issues #2 and #3)

Issue #2: Unprotected Chat Summary Trigger (CRITICAL)
- Add authentication and admin authorization to /trigger endpoint
- Add path validation to prevent directory traversal
- Block access to system directories (/etc, /root, /sys, /proc, /boot)
- Add tests for unauthorized access and invalid paths

Issue #3: Exception Information Disclosure (HIGH)
- Replace raw exception messages with generic errors
- Log full exception details server-side with logger.exception()
- Affected files: auth.py, files.py, passkey.py, opc_agents.py
- Prevents information leakage about system architecture

Security Impact:
- Prevents unauthenticated file system writes
- Reduces reconnaissance opportunities for attackers
- Maintains security while preserving debugging capability

Tests: 214 passing, all security tests verified

dashboard/backend/routers/auth.py

@@ -283,9 +283,8 @@ async def save_preferences(request: Request, current_user=Depends(auth.get_curre
     except HTTPException:
         raise
     except Exception as e:
-        print(f"Error saving preferences: {e}")
-        traceback.print_exc()
-        raise HTTPException(status_code=500, detail=str(e))
+        logger.exception("Error saving preferences for user %s", current_user.username)
+        raise HTTPException(status_code=500, detail="Failed to save preferences")
 
 
 @router.delete("/rbac/overrides/{username}")