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-schema/src/bin/aether-schema.rs

@@ -0,0 +1,76 @@
+use std::path::PathBuf;
+
+use aether_data_schema::dialect::{mysql, postgres, sqlite};
+use aether_data_schema::{check_generated_dir, generate_loaded_to_dir, load_schema_sources};
+use clap::{Parser, Subcommand, ValueEnum};
+
+#[derive(Debug, Parser)]
+#[command(name = "aether-schema")]
+#[command(about = "Generate SQL from Aether logical schema definitions")]
+struct Cli {
+    #[command(subcommand)]
+    command: Command,
+}
+
+#[derive(Debug, Subcommand)]
+enum Command {
+    Generate {
+        #[arg(long, default_value = "crates/aether-data/schema/logical")]
+        schema_dir: PathBuf,
+        #[arg(long, default_value = "crates/aether-data/schema/generated")]
+        output_dir: PathBuf,
+    },
+    Check {
+        #[arg(long, default_value = "crates/aether-data/schema/logical")]
+        schema_dir: PathBuf,
+        #[arg(long, default_value = "crates/aether-data/schema/generated")]
+        output_dir: PathBuf,
+        #[arg(long = "require-tables-from")]
+        require_tables_from: Vec<PathBuf>,
+    },
+    Print {
+        #[arg(long, default_value = "crates/aether-data/schema/logical")]
+        schema_dir: PathBuf,
+        #[arg(long)]
+        driver: Driver,
+    },
+}
+
+#[derive(Debug, Clone, Copy, ValueEnum)]
+enum Driver {
+    Postgres,
+    Mysql,
+    Sqlite,
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let cli = Cli::parse();
+    match cli.command {
+        Command::Generate {
+            schema_dir,
+            output_dir,
+        } => {
+            let loaded = load_schema_sources(schema_dir)?;
+            generate_loaded_to_dir(&loaded, output_dir)?;
+        }
+        Command::Check {
+            schema_dir,
+            output_dir,
+            require_tables_from,
+        } => {
+            let loaded = load_schema_sources(schema_dir)?;
+            check_generated_dir(&loaded, output_dir)?;
+            aether_data_schema::check_required_tables(&loaded.schema, &require_tables_from)?;
+        }
+        Command::Print { schema_dir, driver } => {
+            let schema = load_schema_sources(schema_dir)?.schema;
+            let output = match driver {
+                Driver::Postgres => postgres::emit_schema(&schema),
+                Driver::Mysql => mysql::emit_schema(&schema),
+                Driver::Sqlite => sqlite::emit_schema(&schema),
+            };
+            print!("{output}");
+        }
+    }
+    Ok(())
+}