fix: sanitize FTS5 queries and close mirror DB connections

Two bugs fixed:

1. search_messages() crashes with OperationalError when user queries
   contain FTS5 special characters (+, ", (, {, dangling AND/OR, etc).
   Added _sanitize_fts5_query() to strip dangerous operators and a
   fallback try-except for edge cases.

2. _append_to_sqlite() in mirror.py creates a new SessionDB per call
   but never closes it, leaking SQLite connections. Added finally block
   to ensure db.close() is always called.

hermes_state.py

@@ -16,6 +16,7 @@ Key design decisions:
 
 import json
 import os
+import re
 import sqlite3
 import time
 from pathlib import Path
@@ -322,6 +323,32 @@ class SessionDB:
     # Search
     # =========================================================================
 
+    @staticmethod
+    def _sanitize_fts5_query(query: str) -> str:
+        """Sanitize user input for safe use in FTS5 MATCH queries.
+
+        FTS5 has its own query syntax where characters like ``"``, ``(``, ``)``,
+        ``+``, ``*``, ``{``, ``}`` and bare boolean operators (``AND``, ``OR``,
+        ``NOT``) have special meaning.  Passing raw user input directly to
+        MATCH can cause ``sqlite3.OperationalError``.
+
+        Strategy: strip characters that are only meaningful as FTS5 operators
+        and would otherwise cause syntax errors.  This preserves normal keyword
+        search while preventing crashes on inputs like ``C++``, ``"unterminated``,
+        or ``hello AND``.
+        """
+        # Remove FTS5-special characters that are not useful in keyword search
+        sanitized = re.sub(r'[+{}()"^]', " ", query)
+        # Collapse repeated * (e.g. "***") into a single one, and remove
+        # leading * (prefix-only matching requires at least one char before *)
+        sanitized = re.sub(r"\*+", "*", sanitized)
+        sanitized = re.sub(r"(^|\s)\*", r"\1", sanitized)
+        # Remove dangling boolean operators at start/end that would cause
+        # syntax errors (e.g. "hello AND" or "OR world")
+        sanitized = re.sub(r"(?i)^(AND|OR|NOT)\b\s*", "", sanitized.strip())
+        sanitized = re.sub(r"(?i)\s+(AND|OR|NOT)\s*$", "", sanitized.strip())
+        return sanitized.strip()
+
     def search_messages(
         self,
         query: str,
@@ -345,6 +372,10 @@ class SessionDB:
         if not query or not query.strip():
             return []
 
+        query = self._sanitize_fts5_query(query)
+        if not query:
+            return []
+
         if source_filter is None:
             source_filter = ["cli", "telegram", "discord", "whatsapp", "slack"]
 
@@ -384,7 +415,11 @@ class SessionDB:
             LIMIT ? OFFSET ?
         """
 
-        cursor = self._conn.execute(sql, params)
+        try:
+            cursor = self._conn.execute(sql, params)
+        except sqlite3.OperationalError:
+            # FTS5 query syntax error despite sanitization — return empty
+            return []
         matches = [dict(row) for row in cursor.fetchall()]
 
         # Add surrounding context (1 message before + after each match)