feat(gateway): add system.sessionAnalytics usage snapshot RPC

src/session/store.ts

@@ -12,6 +12,26 @@ export function parseDuration(s: string): number | null {
   return unit === 'h' ? Number(n) * 3600_000 : Number(n) * 86_400_000;
 }
 
+export interface SessionDailyAnalyticsRow {
+  day: string;
+  sessions: number;
+  messages: number;
+}
+
+export interface SessionTopAnalyticsRow {
+  sessionId: string;
+  messages: number;
+  lastActivity: number;
+}
+
+export interface SessionAnalyticsSnapshot {
+  daily: SessionDailyAnalyticsRow[];
+  topSessions: SessionTopAnalyticsRow[];
+  averageMessagesPerSession: number;
+  totalSessions: number;
+  totalMessages: number;
+}
+
 export class SessionStore {
   private db: Database.Database;
 
@@ -267,4 +287,66 @@ export class SessionStore {
   updateMessageMetadata(messageId: number, metadata: HistoryMetadata): void {
     this.db.prepare('UPDATE messages SET metadata = ? WHERE id = ?').run(JSON.stringify(metadata), messageId);
   }
+
+  getSessionAnalytics(opts: { sinceTimestamp: number; topLimit?: number }): SessionAnalyticsSnapshot {
+    const since = opts.sinceTimestamp;
+    const topLimit = opts.topLimit ?? 10;
+
+    const dailyRows = this.db.prepare(`
+      SELECT
+        date(created_at, 'unixepoch') AS day,
+        COUNT(DISTINCT session_id) AS sessions,
+        COUNT(*) AS messages
+      FROM messages
+      WHERE created_at >= ?
+      GROUP BY day
+      ORDER BY day DESC
+    `).all(since) as Array<{ day: string; sessions: number; messages: number }>;
+
+    const topRows = this.db.prepare(`
+      SELECT
+        session_id,
+        COUNT(*) AS messages,
+        MAX(created_at) AS last_activity
+      FROM messages
+      WHERE created_at >= ?
+      GROUP BY session_id
+      ORDER BY messages DESC, last_activity DESC
+      LIMIT ?
+    `).all(since, topLimit) as Array<{ session_id: string; messages: number; last_activity: number }>;
+
+    const totalMessagesRow = this.db.prepare(`
+      SELECT COUNT(*) AS total_messages
+      FROM messages
+      WHERE created_at >= ?
+    `).get(since) as { total_messages: number };
+
+    const totalSessionsRow = this.db.prepare(`
+      SELECT COUNT(DISTINCT session_id) AS total_sessions
+      FROM messages
+      WHERE created_at >= ?
+    `).get(since) as { total_sessions: number };
+
+    const totalMessages = totalMessagesRow.total_messages ?? 0;
+    const totalSessions = totalSessionsRow.total_sessions ?? 0;
+    const averageMessagesPerSession = totalSessions > 0
+      ? Math.round((totalMessages / totalSessions) * 100) / 100
+      : 0;
+
+    return {
+      daily: dailyRows.map((row) => ({
+        day: row.day,
+        sessions: row.sessions,
+        messages: row.messages,
+      })),
+      topSessions: topRows.map((row) => ({
+        sessionId: row.session_id,
+        messages: row.messages,
+        lastActivity: row.last_activity,
+      })),
+      averageMessagesPerSession,
+      totalSessions,
+      totalMessages,
+    };
+  }
 }