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/build.rs

@@ -0,0 +1,46 @@
+use std::env;
+use std::error::Error;
+use std::fs;
+use std::path::PathBuf;
+
+fn main() {
+    if let Err(err) = build_empty_database_snapshot() {
+        panic!("failed to build empty database snapshot: {err}");
+    }
+}
+
+fn build_empty_database_snapshot() -> Result<(), Box<dyn Error>> {
+    let manifest = manifest_path();
+    let source_root = manifest.parent().ok_or_else(|| {
+        std::io::Error::other("bootstrap manifest should have a parent directory")
+    })?;
+    let output = out_dir()?.join("empty_database_snapshot.sql");
+
+    println!("cargo:rerun-if-changed={}", manifest.display());
+
+    let mut snapshot = Vec::new();
+    let manifest_source = fs::read_to_string(&manifest)?;
+    for part in manifest_source.lines().map(str::trim) {
+        if part.is_empty() || part.starts_with('#') {
+            continue;
+        }
+
+        let fragment = source_root.join(part);
+        println!("cargo:rerun-if-changed={}", fragment.display());
+        snapshot.extend_from_slice(&fs::read(&fragment)?);
+    }
+
+    fs::write(output, snapshot)?;
+    Ok(())
+}
+
+fn manifest_path() -> PathBuf {
+    PathBuf::from(env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR should be set"))
+        .join("schema/bootstrap/postgres/manifest.txt")
+}
+
+fn out_dir() -> Result<PathBuf, Box<dyn Error>> {
+    Ok(PathBuf::from(env::var("OUT_DIR").map_err(|_| {
+        std::io::Error::other("OUT_DIR should be set")
+    })?))
+}