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/ai-coder/agents/chat-retention.md

@@ -0,0 +1,44 @@
+# Conversation Data Retention
+
+Coder Agents automatically cleans up old conversation data to manage database
+growth. Archived conversations and their associated files are periodically
+purged based on a configurable retention period.
+
+## How it works
+
+A background process runs approximately every 10 minutes to remove expired
+conversation data. Only archived conversations are eligible for deletion —
+active (non-archived) conversations are never purged.
+
+When an archived conversation exceeds the retention period, it is deleted along
+with its messages, diff statuses, and queued messages via cascade. Orphaned
+files (not referenced by any active or recently-archived conversation) are also
+deleted. Both operations run in batches of 1,000 rows per cycle.
+
+## Configuration
+
+Navigate to **Deployment Settings** > **Agents** > **Behavior** to configure
+the conversation retention period. The default is 30 days. Use the toggle to
+disable retention entirely.
+
+The retention period is stored as the `agents_chat_retention_days` key in the
+`site_configs` table and can also be managed via the API at
+`/api/experimental/chats/config/retention-days`.
+
+## What gets deleted
+
+| Data                   | Condition                                                                                      | Cascade                                                       |
+|------------------------|------------------------------------------------------------------------------------------------|---------------------------------------------------------------|
+| Archived conversations | Archived longer than retention period                                                          | Messages, diff statuses, queued messages deleted via CASCADE. |
+| Conversation files     | Older than retention period AND not referenced by any active or recently-archived conversation | —                                                             |
+
+## Unarchive safety
+
+If a user unarchives a conversation whose files were purged, stale file
+references are automatically cleaned up by FK cascades. The conversation
+remains usable but previously attached files are no longer available.
+
+## Related links
+
+- [Coder Agents](./index.md)
+- [Data Retention](../../admin/setup/data-retention.md)

docs/manifest.json

@@ -1264,6 +1264,12 @@
 							"description": "Programmatic access to Coder Agents via the experimental Chats API",
 							"path": "./ai-coder/agents/chats-api.md",
 							"state": ["early access"]
+						},
+						{
+							"title": "Chat Data Retention",
+							"description": "Automatic cleanup of old chat data",
+							"path": "./ai-coder/agents/chat-retention.md",
+							"state": ["early access"]
 						}
 					]
 				}

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