Add c.db.transaction((tx) => ...) for safe actor-local SQLite transactions

[NathanFlurry]

Problem

Today transactions are expressed manually with raw SQL:

await c.db.execute("BEGIN");
try {
  // ...queries...
  await c.db.execute("COMMIT");
} catch (error) {
  await c.db.execute("ROLLBACK");
  throw error;
}

c.db is a shared actor-local database handle, so other c.db.execute(...) calls can interleave while a transaction is open (for example from concurrent actions). This makes transaction safety easy to get wrong.

Proposal

Add a first-class transaction API:

await c.db.transaction(async (tx) => {
  await tx.execute("INSERT INTO todos (title) VALUES (?)", title);
  await tx.execute(
    "INSERT INTO comments (todo_id, body) VALUES (last_insert_rowid(), ?)",
    body,
  );
});

Expected behavior

• transaction acquires exclusive access for the callback duration.
• Other c.db operations are queued until transaction commit/rollback.
• Automatically COMMIT on success.
• Automatically ROLLBACK on throw.
• tx only exposes transaction-safe operations (e.g. tx.execute).

Why this helps

• Prevents interleaving bugs with BEGIN/COMMIT.
• Makes docs/examples safer and simpler.
• Aligns with ergonomics from other SQLite libraries.

Optional follow-ups

• Decide nested transaction behavior (SAVEPOINT or disallow nesting).
• Add typed return support from callback (transaction<T>(...) => Promise<T>).

[linear]

RVT-5955