Add multi-database data layer

Introduce aether-data-schema and driver-specific schema generation for Postgres, MySQL, and SQLite.

Split data backends, lifecycle, repositories, and gateway runtime integration across database drivers.

Verified with cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace.

crates/aether-data/src/driver/redis/namespace.rs

@@ -0,0 +1,43 @@
+use aether_cache::CacheKeyNamespace;
+
+use crate::driver::redis::{RedisLockKey, RedisStreamName};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct RedisKeyspace {
+    namespace: CacheKeyNamespace,
+}
+
+impl RedisKeyspace {
+    pub fn new(prefix: Option<&str>) -> Self {
+        let normalized = prefix.unwrap_or_default().trim().trim_matches(':');
+        Self {
+            namespace: CacheKeyNamespace::new(normalized),
+        }
+    }
+
+    pub fn key(&self, raw_key: &str) -> String {
+        self.namespace.key(raw_key)
+    }
+
+    pub fn lock_key(&self, raw_key: &str) -> RedisLockKey {
+        RedisLockKey(self.namespace.child("lock").key(raw_key))
+    }
+
+    pub fn stream_name(&self, raw_name: &str) -> RedisStreamName {
+        RedisStreamName(self.namespace.child("stream").key(raw_name))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::RedisKeyspace;
+
+    #[test]
+    fn composes_prefixed_lock_and_stream_names() {
+        let keyspace = RedisKeyspace::new(Some("aether"));
+
+        assert_eq!(keyspace.key("auth:user"), "aether:auth:user");
+        assert_eq!(keyspace.lock_key("poller").0, "aether:lock:poller");
+        assert_eq!(keyspace.stream_name("audit").0, "aether:stream:audit");
+    }
+}