batch3 T1 review fixes: Pane discriminated union + index naming

- Restructure Pane as a tagged union over kind so checking kind narrows state
- Rename session_panes_session_idx -> idx_session_panes_session for naming
  consistency with idx_sessions_project, idx_messages_session

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

apps/server/src/schema.sql

@@ -56,4 +56,4 @@ CREATE TABLE IF NOT EXISTS session_panes (
   created_at   TIMESTAMPTZ NOT NULL DEFAULT clock_timestamp(),
   UNIQUE (session_id, position)
 );
-CREATE INDEX IF NOT EXISTS session_panes_session_idx ON session_panes (session_id);
+CREATE INDEX IF NOT EXISTS idx_session_panes_session ON session_panes (session_id);

apps/server/src/types/api.ts

@@ -91,15 +91,18 @@ export type ChatPaneState = Record<string, never>;
 
 export type PaneState = ChatPaneState | FileBrowserPaneState;
 
-export interface Pane {
+interface PaneBase {
   id: string;
   session_id: string;
   position: number;
-  kind: PaneKind;
-  state: PaneState;
   created_at: string;
 }
 
+export type Pane = PaneBase & (
+  | { kind: 'chat'; state: ChatPaneState }
+  | { kind: 'file_browser'; state: FileBrowserPaneState }
+);
+
 export interface PaneCreateRequest {
   kind: PaneKind;
   position?: number;  // optional; if omitted, append at end