Persist bot mode across restarts via database

Adds a BotSettings key-value table. The active mode is saved
when changed via /bcs-mode and restored on startup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

utils/database.py

@@ -126,6 +126,15 @@ class Database:
                 ALTER TABLE UserState ADD UserNotes NVARCHAR(MAX) NULL
         """)
 
+        cursor.execute("""
+            IF NOT EXISTS (SELECT * FROM sys.tables WHERE name = 'BotSettings')
+            CREATE TABLE BotSettings (
+                SettingKey   NVARCHAR(100) NOT NULL PRIMARY KEY,
+                SettingValue NVARCHAR(MAX) NULL,
+                UpdatedAt    DATETIME2 NOT NULL DEFAULT SYSUTCDATETIME()
+            )
+        """)
+
         cursor.execute("""
             IF NOT EXISTS (SELECT * FROM sys.tables WHERE name = 'LlmLog')
             CREATE TABLE LlmLog (
@@ -414,6 +423,57 @@ class Database:
         finally:
             conn.close()
 
+    # ------------------------------------------------------------------
+    # Bot Settings (key-value store)
+    # ------------------------------------------------------------------
+    async def save_setting(self, key: str, value: str) -> None:
+        if not self._available:
+            return
+        try:
+            await asyncio.to_thread(self._save_setting_sync, key, value)
+        except Exception:
+            logger.exception("Failed to save setting %s", key)
+
+    def _save_setting_sync(self, key: str, value: str):
+        conn = self._connect()
+        try:
+            cursor = conn.cursor()
+            cursor.execute(
+                """MERGE BotSettings AS target
+                   USING (SELECT ? AS SettingKey) AS source
+                   ON target.SettingKey = source.SettingKey
+                   WHEN MATCHED THEN
+                       UPDATE SET SettingValue = ?, UpdatedAt = SYSUTCDATETIME()
+                   WHEN NOT MATCHED THEN
+                       INSERT (SettingKey, SettingValue) VALUES (?, ?);""",
+                key, value, key, value,
+            )
+            cursor.close()
+        finally:
+            conn.close()
+
+    async def load_setting(self, key: str, default: str | None = None) -> str | None:
+        if not self._available:
+            return default
+        try:
+            return await asyncio.to_thread(self._load_setting_sync, key, default)
+        except Exception:
+            logger.exception("Failed to load setting %s", key)
+            return default
+
+    def _load_setting_sync(self, key: str, default: str | None) -> str | None:
+        conn = self._connect()
+        try:
+            cursor = conn.cursor()
+            cursor.execute(
+                "SELECT SettingValue FROM BotSettings WHERE SettingKey = ?", key
+            )
+            row = cursor.fetchone()
+            cursor.close()
+            return row[0] if row else default
+        finally:
+            conn.close()
+
     async def close(self):
         """No persistent connection to close (connections are per-operation)."""
         pass