feat: add per-user retention settings to users.toml

New [user.retention] TOML sub-block with auto_close_days and
auto_delete_days. Both default to 0, which disables the respective
sweep — a conservative default that preserves current behaviour for
existing deployments and lets admins test auto-close in isolation.

Only parsing + the test User literals are touched here; the actual
lazy sweep consuming these values lands in the next commit.

server/tests/analyze_test.rs

@@ -31,6 +31,7 @@ fn make_user(slug: &str) -> User {
         web_password: bcrypt::hash("s", 4).unwrap(),
         role: "doctor".into(),
         whisper: Default::default(),
+        retention: Default::default(),
     }
 }