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/config.rs

@@ -1,15 +1,44 @@
-use crate::postgres::PostgresPoolConfig;
-use crate::redis::RedisClientConfig;
+use crate::database::SqlDatabaseConfig;
+use crate::driver::postgres::PostgresPoolConfig;
+use crate::driver::redis::RedisClientConfig;
 use crate::DataLayerError;
 
 #[derive(Debug, Clone, Default, serde::Serialize, serde::Deserialize, PartialEq, Eq)]
 pub struct DataLayerConfig {
+    pub database: Option<SqlDatabaseConfig>,
     pub postgres: Option<PostgresPoolConfig>,
     pub redis: Option<RedisClientConfig>,
 }
 
 impl DataLayerConfig {
+    pub fn from_database(database: SqlDatabaseConfig) -> Self {
+        Self {
+            database: Some(database),
+            postgres: None,
+            redis: None,
+        }
+    }
+
+    pub fn from_postgres(postgres: PostgresPoolConfig) -> Self {
+        Self {
+            database: Some(SqlDatabaseConfig::from_postgres_config(postgres)),
+            postgres: None,
+            redis: None,
+        }
+    }
+
+    pub fn effective_database(&self) -> Option<SqlDatabaseConfig> {
+        self.database.clone().or_else(|| {
+            self.postgres
+                .clone()
+                .map(SqlDatabaseConfig::from_postgres_config)
+        })
+    }
+
     pub fn validate(&self) -> Result<(), DataLayerError> {
+        if let Some(database) = &self.database {
+            database.validate()?;
+        }
         if let Some(postgres) = &self.postgres {
             postgres.validate()?;
         }
@@ -20,19 +49,21 @@ impl DataLayerConfig {
     }
 
     pub fn has_persistent_backends(&self) -> bool {
-        self.postgres.is_some() || self.redis.is_some()
+        self.effective_database().is_some() || self.redis.is_some()
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::DataLayerConfig;
-    use crate::postgres::PostgresPoolConfig;
-    use crate::redis::RedisClientConfig;
+    use crate::database::{DatabaseDriver, SqlDatabaseConfig, SqlPoolConfig};
+    use crate::driver::postgres::PostgresPoolConfig;
+    use crate::driver::redis::RedisClientConfig;
 
     #[test]
     fn validates_nested_backend_configs() {
         let config = DataLayerConfig {
+            database: None,
             postgres: Some(PostgresPoolConfig {
                 database_url: "postgres://localhost/aether".to_string(),
                 min_connections: 2,
@@ -56,6 +87,7 @@ mod tests {
     #[test]
     fn rejects_invalid_nested_backend_configs() {
         let config = DataLayerConfig {
+            database: None,
             postgres: Some(PostgresPoolConfig {
                 database_url: String::new(),
                 min_connections: 4,
@@ -71,4 +103,31 @@ mod tests {
 
         assert!(config.validate().is_err());
     }
+
+    #[test]
+    fn new_database_config_takes_priority_over_legacy_postgres_config() {
+        let config = DataLayerConfig {
+            database: Some(SqlDatabaseConfig {
+                driver: DatabaseDriver::Sqlite,
+                url: "sqlite://./data/aether.db".to_string(),
+                pool: SqlPoolConfig::default(),
+            }),
+            postgres: Some(PostgresPoolConfig {
+                database_url: "postgres://localhost/aether".to_string(),
+                min_connections: 1,
+                max_connections: 4,
+                acquire_timeout_ms: 1_000,
+                idle_timeout_ms: 5_000,
+                max_lifetime_ms: 30_000,
+                statement_cache_capacity: 64,
+                require_ssl: false,
+            }),
+            redis: None,
+        };
+
+        let effective = config
+            .effective_database()
+            .expect("database config should exist");
+        assert_eq!(effective.driver, DatabaseDriver::Sqlite);
+    }
 }