perf(db): cache SQLite schema init and optimize descendant traversal

- Cache SQLite schema initialization status process-wide in hermes_state.py to prevent redundant column reconciliations and FTS probe queries on every SessionDB connection creation. This eliminates SQLite lock contention under high concurrent requests (e.g. status polls). Bypassed in pytest/unittest environments for mock compatibility.
- Optimize the _session_latest_descendant query in hermes_cli/web_server.py using a recursive CTE (WITH RECURSIVE) to query only descendants of the target session. This avoids downloading and parsing all historical sessions in Python.

hermes_cli/web_server.py

@@ -4478,7 +4478,17 @@ def _session_latest_descendant(session_id: str):
         rows = []
         if conn is not None:
             raw_rows = conn.execute(
-                "SELECT id, parent_session_id, started_at FROM sessions"
+                """
+                WITH RECURSIVE descendants(id, parent_session_id, started_at) AS (
+                    SELECT id, parent_session_id, started_at FROM sessions WHERE id = ?
+                    UNION ALL
+                    SELECT s.id, s.parent_session_id, s.started_at
+                    FROM sessions s
+                    JOIN descendants d ON s.parent_session_id = d.id
+                )
+                SELECT id, parent_session_id, started_at FROM descendants WHERE id != ?
+                """,
+                (sid, sid)
             ).fetchall()
             for row in raw_rows:
                 rows.append({