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.

coderd/database/queries/chatfiles.sql

@@ -18,3 +18,37 @@ FROM chat_files cf
 JOIN chat_file_links cfl ON cfl.file_id = cf.id
 WHERE cfl.chat_id = @chat_id::uuid
 ORDER BY cf.created_at ASC;
+
+-- TODO(cian): Add indexes on chats(archived, updated_at) and
+-- chat_files(created_at) for purge query performance.
+-- See: https://github.com/coder/internal/issues/1438
+-- name: DeleteOldChatFiles :execrows
+-- Deletes chat files that are older than the given threshold and are
+-- not referenced by any chat that is still active or was archived
+-- within the same threshold window. This covers two cases:
+-- 1. Orphaned files not linked to any chat.
+-- 2. Files whose every referencing chat has been archived for longer
+--    than the retention period.
+WITH kept_file_ids AS (
+    -- NOTE: This uses updated_at as a proxy for archive time
+    -- because there is no archived_at column. Correctness
+    -- requires that updated_at is never backdated on archived
+    -- chats. See ArchiveChatByID.
+    SELECT DISTINCT cfl.file_id
+    FROM chat_file_links cfl
+    JOIN chats c ON c.id = cfl.chat_id
+    WHERE c.archived = false
+       OR c.updated_at >= @before_time::timestamptz
+),
+deletable AS (
+    SELECT cf.id
+    FROM chat_files cf
+    LEFT JOIN kept_file_ids k ON cf.id = k.file_id
+    WHERE cf.created_at < @before_time::timestamptz
+      AND k.file_id IS NULL
+    ORDER BY cf.created_at ASC
+    LIMIT @limit_count
+)
+DELETE FROM chat_files
+USING deletable
+WHERE chat_files.id = deletable.id;