API protection limit content size (#4583)
API protection limit content size (#4583)

* API protection limit content size
We had no limit to the length of an article sent through our GReader API to clients, which was a problem:

The article https://devblogs.microsoft.com/dotnet/performance_improvements_in_net_7/
from the feed https://devblogs.microsoft.com/dotnet/feed/
is about 700KB long, which makes some clients (tested with News+) fail.

We should try other clients to verify that 500KB is a safe enough limit.

* Light optimisation

* Use class constant
aledeg/FreshRSS,Alkarex/FreshRSS,Alkarex/FreshRSS,FreshRSS/FreshRSS,Alkarex/FreshRSS,Alkarex/FreshRSS,aledeg/FreshRSS,FreshRSS/FreshRSS,aledeg/FreshRSS,FreshRSS/FreshRSS,aledeg/FreshRSS,FreshRSS/FreshRSS