feat(db): schema bootstrap with append-only audit_log table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/db/src/lib.rs

@@ -37,4 +37,13 @@ impl Db {
 
         Ok(())
     }
+
+    /// Apply all pending schema migrations (embedded at compile time).
+    ///
+    /// Pre-1.0 the migration files are rewritten freely and dev databases are
+    /// recreated; this is the schema-bootstrap mechanism, not forward-migration
+    /// discipline.
+    pub async fn migrate(&self) -> Result<(), sqlx::migrate::MigrateError> {
+        sqlx::migrate!().run(&self.pool).await
+    }
 }