Fetch a restore's history in one request, and let RUST_LOG through

Now that a scroll anchor is a seq, the restore knows exactly how far back
it has to reach, so it asks for that span in one request instead of
walking there a page at a time. ai-app-2 suggested it; the arithmetic is
theirs. `read_window` counts lines and a transcript numbers them one per
seq, so the distance to the anchor is the number of events to ask for --
and were seqs ever sparse, that difference is larger than the count,
which overshoots into older history rather than stopping short.

Capped at [RESTORE_PAGE_MAX], and the loop already there is what makes
the cap safe: a span past it comes back in several requests rather than
one, which is what every restore did until now. The bytes are the same
either way -- every row between the anchor and the newest end has to be
loaded for the list to be able to count to it -- so this only trades
round trips against response size.

Measured on a 16,133-event session, restoring to seq 2000 (14,133 events
back, the extreme case): **19 requests before, 5 after.** The realistic
case, a couple of thousand events back, is 4 before and 2 after. At
`--delay 150` the deep one puts the row on screen at 5.7s and the
moderate one at 2.5s, and in both the row does not move once it lands.

Also: `RUST_LOG` did nothing. `with_env_filter("info")` is a fixed
directive that never reads the environment, so the per-request page
diagnostics AGENTS.md tells you to turn on with `RUST_LOG=ai_server=debug`
printed nothing at all -- which reads as the code you are instrumenting
being wrong rather than as the switch being disconnected. It is a
fallback now, so the default is still `info`. Those diagnostics are what
the request counts above were measured with.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/src/main.rs

@@ -132,7 +132,17 @@ async fn main() -> Result<()> {
         .install_default()
         .expect("no other TLS crypto provider is installed before main");
 
-    tracing_subscriber::fmt().with_env_filter("info").init();
+    // `info` unless RUST_LOG says otherwise. Written as a *fallback* rather than as the filter,
+    // because `with_env_filter("info")` is a fixed directive that never reads the environment --
+    // so the per-request diagnostics that AGENTS.md tells you to turn on with
+    // `RUST_LOG=ai_server=debug` printed nothing, and the switch looked like the code it was
+    // meant to instrument being wrong.
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            tracing_subscriber::EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("info")),
+        )
+        .init();
     let args = Args::parse();
 
     let config_path = args