fix: harden concurrency limits and high-RPM runtime paths

Bound request, stream, queue, and shutdown resource lifetimes. Reduce scheduler and Redis hot-path work and isolate database maintenance. Include regression coverage, load probes, and concurrency audit results.

crates/aether-runtime/state/src/score_window.rs

@@ -0,0 +1,25 @@
+/// Maximum number of members examined by one server-side window aggregation.
+pub const SCORE_WINDOW_AGGREGATION_MEMBER_LIMIT: usize = 512;
+
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
+pub struct ScoreWindowU64Stats {
+    pub sum: u64,
+    pub positive_count: u64,
+}
+
+impl ScoreWindowU64Stats {
+    /// Members encode the value after their final colon. Invalid or zero values
+    /// do not contribute to the positive sample count.
+    pub fn from_members<'a>(members: impl IntoIterator<Item = &'a str>) -> Self {
+        let mut stats = Self::default();
+        for member in members {
+            let value = member
+                .rsplit_once(':')
+                .and_then(|(_, suffix)| suffix.parse::<u64>().ok())
+                .unwrap_or(0);
+            stats.sum = stats.sum.saturating_add(value);
+            stats.positive_count += u64::from(value > 0);
+        }
+        stats
+    }
+}