feat(FN-1146): add configurable AI session cleanup lifecycle

- Add aiSessionTtlMs and aiSessionCleanupIntervalMs project settings with defaults and bounds for cleanup scheduling
- Extend AiSessionStore cleanup to expire stale in-progress sessions, emit deletion events, and support start/stop scheduled cleanup loops
- Wire server startup/shutdown to load cleanup settings and manage the scheduled ai_sessions sweep lifecycle
- Align planning, subtask breakdown, and mission interview in-memory session retention to a 7-day TTL with shared deletion-driven cleanup
- Update schema/tests/docs for migration v15 ai_sessions indexing and end-to-end cleanup/TTL behavior stability

packages/core/src/db.ts

@@ -59,7 +59,7 @@ export function fromJson<T>(json: string | null | undefined): T | undefined {
 
 // ── Schema Definition ────────────────────────────────────────────────
 
-const SCHEMA_VERSION = 14;
+const SCHEMA_VERSION = 15;
 
 function normalizeTaskComments(
   steeringComments: SteeringComment[] | undefined,
@@ -516,6 +516,14 @@ export class Database {
         this.db.exec(`CREATE INDEX IF NOT EXISTS idxAgentRatingsCreatedAt ON agentRatings(createdAt)`);
       });
     }
+
+    if (version < 15) {
+      this.applyMigration(15, () => {
+        if (this.hasTable("ai_sessions")) {
+          this.db.exec(`CREATE INDEX IF NOT EXISTS idxAiSessionsUpdatedAt ON ai_sessions(updatedAt)`);
+        }
+      });
+    }
   }
 
   /**
@@ -530,6 +538,16 @@ export class Database {
       .run(String(targetVersion));
   }
 
+  /**
+   * Check whether a table exists.
+   */
+  private hasTable(table: string): boolean {
+    const row = this.db
+      .prepare("SELECT name FROM sqlite_master WHERE type = 'table' AND name = ?")
+      .get(table) as { name: string } | undefined;
+    return Boolean(row);
+  }
+
   /**
    * Check whether a table has a given column.
    */