chore: replace placeholder package with role-named workspace

crates/db/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "db"
+version = "0.0.0"
+edition.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+sqlx.workspace = true
+thiserror.workspace = true
+
+[dev-dependencies]
+tokio.workspace = true

crates/db/src/lib.rs

@@ -0,0 +1 @@
+//! Database access. All SQL lives in this crate.