AI spam classification: default to 2 KiB of text and a +2.0 cap on the model's tag, as calibrated for low-end CPU-only instances

The calibration harness (tests/src/system/ai_calibration.rs, ignored) sends
what the classifier sends to a real local model and scores its answers with
the classifier's parser.

tests/src/system/mod.rs

@@ -7,6 +7,7 @@
 pub mod antispam;
 pub mod authentication;
 pub mod ai;
+pub mod ai_calibration;
 pub mod authorization;
 pub mod branding;
 pub mod crypto;