batch3 T2 review fix: move PATCH count+bounds check inside sql.begin

A concurrent DELETE between the count read and the transaction could allow
an invalid position value to slip in. Mirror the POST fix by validating
count + bounds inside the transaction.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/server/src/routes/panes.ts

@@ -147,20 +147,18 @@ export function registerPaneRoutes(app: FastifyInstance, sql: Sql): void {
       const sid = pane.session_id;
       const oldPos = pane.position;
 
-      // Validate position if provided
-      if (position !== undefined) {
-        const countRows = await sql<{ n: number }[]>`
-          SELECT COUNT(*)::int AS n FROM session_panes WHERE session_id = ${sid}
-        `;
-        const count = countRows[0]?.n ?? 0;
-        if (position < 0 || position >= count) {
-          reply.code(400);
-          return { error: `position must be between 0 and ${count - 1}` };
-        }
-      }
-
       // Apply position and/or state changes atomically
+      let patchError: string | null = null;
       await sql.begin(async (tx) => {
+        if (position !== undefined) {
+          const countRows = await tx<{ n: number }[]>`
+            SELECT COUNT(*)::int AS n FROM session_panes WHERE session_id = ${sid}
+          `;
+          const count = countRows[0]?.n ?? 0;
+          if (position < 0 || position >= count) {
+            throw `position must be between 0 and ${count - 1}`;
+          }
+        }
         if (position !== undefined && position !== oldPos) {
           await movePane(tx, req.params.id, sid, oldPos, position);
         }
@@ -170,8 +168,19 @@ export function registerPaneRoutes(app: FastifyInstance, sql: Sql): void {
             WHERE id = ${req.params.id}
           `;
         }
+      }).catch((err: unknown) => {
+        if (typeof err === 'string') {
+          patchError = err;
+        } else {
+          throw err;
+        }
       });
 
+      if (patchError !== null) {
+        reply.code(400);
+        return { error: patchError };
+      }
+
       const [updated] = await sql<Pane[]>`
         SELECT id, session_id, position, kind, state, created_at
         FROM session_panes WHERE id = ${req.params.id}