feat: add chat and chat_files cleanup to dbpurge (#23833)

Fixes https://github.com/coder/coder/issues/23910

Adds periodic cleanup of chats and chat files to the dbpurge background
goroutine, with a configurable retention period exposed in the Agent
settings UI.

> 🤖 Written by a Coder Agent. Reviewed by a human.

docs/reference/api/schemas.md

@@ -2025,6 +2025,20 @@ AuthorizationObject can represent a "set" of objects, such as: all workspaces in
 |----------------------|---------|----------|--------------|-------------|
 | `acquire_batch_size` | integer | false    |              |             |
 
+## codersdk.ChatRetentionDaysResponse
+
+```json
+{
+  "retention_days": 0
+}
+```
+
+### Properties
+
+| Name             | Type    | Required | Restrictions | Description |
+|------------------|---------|----------|--------------|-------------|
+| `retention_days` | integer | false    |              |             |
+
 ## codersdk.ConnectionLatency
 
 ```json
@@ -10299,6 +10313,20 @@ Restarts will only happen on weekdays in this list on weeks which line up with W
 | `logo_url`             | string                                                  | false    |              |                                                                     |
 | `service_banner`       | [codersdk.BannerConfig](#codersdkbannerconfig)          | false    |              | Deprecated: ServiceBanner has been replaced by AnnouncementBanners. |
 
+## codersdk.UpdateChatRetentionDaysRequest
+
+```json
+{
+  "retention_days": 0
+}
+```
+
+### Properties
+
+| Name             | Type    | Required | Restrictions | Description |
+|------------------|---------|----------|--------------|-------------|
+| `retention_days` | integer | false    |              |             |
+
 ## codersdk.UpdateCheckResponse
 
 ```json