Refactor Query contract: uniform ergonomics for SQLite + PostgreSQL

- Add `publish_key()` default method to `Query` trait (returns `None`)
- Add `mutation_exec!` macro for SQLite INSERT/UPDATE/DELETE ops; overrides
  `publish_key()` so the mutation carries its own invalidation key
- Add `Store::exec()` which runs a mutation and auto-publishes its key —
  action handlers no longer need a separate `store.publish()` call
- Add `PgQuery` trait + `pg_query_one!` / `pg_query_all!` macros
  (feature-gated on `pg_replication`) mirroring the SQLite Query API
- Add `cx.run_pg(pool, query)` to `RenderContext` which executes a PgQuery
  and auto-subscribes its cache key — no more manual `cx.subscribe()` calls
- Update todo example: replace plain mutation fns with `mutation_exec!` structs
  and `store.exec()` calls; remove `action_then_publish` helper
- Update pg_replication example: add `ListMessages` via `pg_query_all!` and
  replace manual subscribe + raw query with `cx.run_pg()`

https://claude.ai/code/session_01HXe8rAZPweU9j2piBo9MdG

examples/todo/queries.rs

@@ -40,36 +40,42 @@ livevue_rs::query_one!(GetTodo {
 });
 
 // ---------------------------------------------------------------------------
-// Write operations (plain sqlx, not through Query trait)
+// Write operations (mutation structs via mutation_exec!)
 // ---------------------------------------------------------------------------
 //
-// Mutations don't go through cx.run() because they're not reads and don't
-// produce subscription keys. They're called directly by action handlers,
-// which then trigger a full rerender.
+// Each mutation struct implements Query with publish_key() set to the cache
+// key it invalidates. Use Store::exec() in action handlers — it executes the
+// mutation and publishes the key automatically.
 
-/// Insert a new todo. Returns the new row's id.
-pub async fn create_todo(db: &sqlx::SqlitePool, title: &str) -> anyhow::Result<i64> {
-    let result = sqlx::query("INSERT INTO todos (title, completed) VALUES (?, false)")
-        .bind(title)
-        .execute(db)
-        .await?;
-    Ok(result.last_insert_rowid())
+/// Insert a new todo.
+pub struct CreateTodo {
+    pub title: String,
 }
 
+livevue_rs::mutation_exec!(CreateTodo {
+    sql: "INSERT INTO todos (title, completed) VALUES (?, false)",
+    params: [|s: &CreateTodo| s.title.clone()],
+    publish: |_| CacheKey::Table { table: "todos" },
+});
+
 /// Toggle a todo's completed status.
-pub async fn toggle_todo(db: &sqlx::SqlitePool, id: i64) -> anyhow::Result<()> {
-    sqlx::query("UPDATE todos SET completed = NOT completed WHERE id = ?")
-        .bind(id)
-        .execute(db)
-        .await?;
-    Ok(())
+pub struct ToggleTodo {
+    pub id: i64,
 }
 
+livevue_rs::mutation_exec!(ToggleTodo {
+    sql: "UPDATE todos SET completed = NOT completed WHERE id = ?",
+    params: [|s: &ToggleTodo| s.id],
+    publish: |_| CacheKey::Table { table: "todos" },
+});
+
 /// Delete a todo by id.
-pub async fn delete_todo(db: &sqlx::SqlitePool, id: i64) -> anyhow::Result<()> {
-    sqlx::query("DELETE FROM todos WHERE id = ?")
-        .bind(id)
-        .execute(db)
-        .await?;
-    Ok(())
+pub struct DeleteTodo {
+    pub id: i64,
 }
+
+livevue_rs::mutation_exec!(DeleteTodo {
+    sql: "DELETE FROM todos WHERE id = ?",
+    params: [|s: &DeleteTodo| s.id],
+    publish: |_| CacheKey::Table { table: "todos" },
+});