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/maintenance.rs

@@ -0,0 +1,70 @@
+//! Maintenance and aggregation DTOs used by the runtime data layer.
+//!
+//! These are not cross-crate repository contracts, but they are shared across
+//! the backend composition layer and maintenance entrypoints.
+
+use chrono::{DateTime, Utc};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct DatabaseMaintenanceSummary {
+    pub attempted: usize,
+    pub succeeded: usize,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct DatabasePoolSummary {
+    pub driver: crate::database::DatabaseDriver,
+    pub checked_out: usize,
+    pub pool_size: usize,
+    pub idle: usize,
+    pub max_connections: u32,
+    pub usage_rate: f64,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct WalletDailyUsageAggregationInput {
+    pub billing_date: String,
+    pub billing_timezone: String,
+    pub window_start_unix_secs: u64,
+    pub window_end_unix_secs: u64,
+    pub aggregated_at_unix_secs: u64,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct WalletDailyUsageAggregationResult {
+    pub aggregated_wallets: usize,
+    pub deleted_stale_ledgers: usize,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct StatsHourlyAggregationInput {
+    pub target_hour_utc: DateTime<Utc>,
+    pub aggregated_at: DateTime<Utc>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct StatsDailyAggregationInput {
+    pub target_day_utc: DateTime<Utc>,
+    pub aggregated_at: DateTime<Utc>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct StatsDailyAggregationSummary {
+    pub day_start_utc: DateTime<Utc>,
+    pub total_requests: i64,
+    pub model_rows: usize,
+    pub provider_rows: usize,
+    pub api_key_rows: usize,
+    pub error_rows: usize,
+    pub user_rows: usize,
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct StatsHourlyAggregationSummary {
+    pub hour_utc: DateTime<Utc>,
+    pub total_requests: i64,
+    pub user_rows: usize,
+    pub user_model_rows: usize,
+    pub model_rows: usize,
+    pub provider_rows: usize,
+}