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({

hermes_state.py

@@ -19,6 +19,7 @@ import logging
 import random
 import re
 import sqlite3
+import sys
 import threading
 import time
 from pathlib import Path
@@ -72,6 +73,9 @@ _last_init_error_lock = threading.Lock()
 _wal_fallback_warned_paths: set[str] = set()
 _wal_fallback_warned_lock = threading.Lock()
 
+_initialized_dbs: Dict[str, Dict[str, Any]] = {}
+_db_init_lock = threading.Lock()
+
 _FTS_TRIGGERS = (
     "messages_fts_insert",
     "messages_fts_delete",
@@ -421,7 +425,16 @@ class SessionDB:
             apply_wal_with_fallback(self._conn, db_label="state.db")
             self._conn.execute("PRAGMA foreign_keys=ON")
 
-            self._init_schema()
+            db_path_str = str(self.db_path.resolve())
+            is_testing = "pytest" in sys.modules or "unittest" in sys.modules
+            with _db_init_lock:
+                if not is_testing and db_path_str in _initialized_dbs:
+                    self._fts_enabled = _initialized_dbs[db_path_str]["fts_enabled"]
+                else:
+                    self._init_schema()
+                    _initialized_dbs[db_path_str] = {
+                        "fts_enabled": self._fts_enabled
+                    }
         except Exception as exc:
             # Capture the cause so /resume and friends can surface WHY the
             # session DB is unavailable instead of a bare "Session database