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

src/lib.rs

@@ -15,9 +15,18 @@ pub mod server;
 #[cfg(feature = "pg_replication")]
 pub mod pg_replication;
 
+/// PostgreSQL query trait and macros (`pg_query_one!`, `pg_query_all!`).
+///
+/// Use `cx.run_pg(pool, query)` in render functions to execute a `PgQuery`
+/// and auto-subscribe to its cache key, exactly like `cx.run()` for SQLite.
+#[cfg(feature = "pg_replication")]
+pub mod pg_query;
+
 pub use signal::{Signal, SignalStore, SignalOpts, mangle, client_only, global, data_signals};
 pub use query::{CacheKey, Query};
 pub use store::{Store, KVStore};
+#[cfg(feature = "pg_replication")]
+pub use pg_query::PgQuery;
 pub use context::{RenderContext, ComponentTree, ComponentNode};
 pub use connection::{ConnectionManager, ConnectionState, SubscriptionRegistry};
 pub use sse::{format_patch_elements, format_patch_signals};