Explain: shorter answers, streamed, remembered, and prepared for settings

ai-explain spec, amendment 1 (EX-22 to EX-28):
- answers are three or four sentences, max_tokens 160, cut at 700 chars;
- POST /api/explain streams the answer as server-sent events;
- each node remembers answers in memory (1,000, 24 h), keyed by the facts,
  prompt version and model, shared by server-level administrators;
- resources/explain/settings.json.gz ships answers for settings at their
  defaults, generated with prepare_setting_explanations (717 for 2026.9.27);
- the system prompt no longer carries the per-request marker, so a model
  server can reuse it;
- inbuxa:Explanation gains source, answeredAt and preparedFor.

crates/features/Cargo.toml

@@ -15,6 +15,7 @@ utils = { path = "../utils" }
 ahash = { version = "0.8.12", features = ["serde"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
+xxhash-rust = { version = "0.8.18", features = ["xxh3"] }
 base64 = "0.23"
 
 [dev-dependencies]