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 Files whose every referencing chat has been archived for longer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;

coderd/database/queries/chats.sql

@@ -10,9 +10,14 @@ FROM chats
 ORDER BY (id = @id::uuid) DESC, created_at ASC, id ASC;
 
 -- name: UnarchiveChatByID :many
+-- Unarchives a chat (and its children). Stale file references are
+-- handled automatically by FK cascades on chat_file_links: when
+-- dbpurge deletes a chat_files row, the corresponding
+-- chat_file_links rows are cascade-deleted by PostgreSQL.
 WITH chats AS (
-    UPDATE chats
-    SET archived = false, updated_at = NOW()
+    UPDATE chats SET
+        archived = false,
+        updated_at = NOW()
     WHERE id = @id::uuid OR root_chat_id = @id::uuid
     RETURNING *
 )
@@ -1220,3 +1225,22 @@ LIMIT 1;
 UPDATE chats
 SET last_read_message_id = @last_read_message_id::bigint
 WHERE id = @id::uuid;
+
+-- name: DeleteOldChats :execrows
+-- Deletes chats that have been archived for longer than the given
+-- threshold. Active (non-archived) chats are never deleted.
+-- Related chat_messages, chat_diff_statuses, and
+-- chat_queued_messages are removed via ON DELETE CASCADE.
+-- Parent/root references on child chats are SET NULL.
+WITH deletable AS (
+    SELECT id
+    FROM chats
+    WHERE archived = true
+      AND updated_at < @before_time::timestamptz
+    ORDER BY updated_at ASC
+    LIMIT @limit_count
+)
+DELETE FROM chats
+USING deletable
+WHERE chats.id = deletable.id
+  AND chats.archived = true;

coderd/database/queries/siteconfig.sql

@@ -236,3 +236,20 @@ VALUES ('agents_workspace_ttl', @workspace_ttl::text)
 ON CONFLICT (key) DO UPDATE
 SET value = @workspace_ttl::text
 WHERE site_configs.key = 'agents_workspace_ttl';
+
+-- name: GetChatRetentionDays :one
+-- Returns the chat retention period in days. Chats archived longer
+-- than this and orphaned chat files older than this are purged by
+-- dbpurge. Returns 30 (days) when no value has been configured.
+-- A value of 0 disables chat purging entirely.
+SELECT COALESCE(
+    (SELECT value::integer FROM site_configs
+     WHERE key = 'agents_chat_retention_days'),
+    30
+) :: integer AS retention_days;
+
+-- name: UpsertChatRetentionDays :exec
+INSERT INTO site_configs (key, value)
+VALUES ('agents_chat_retention_days', CAST(@retention_days AS integer)::text)
+ON CONFLICT (key) DO UPDATE SET value = CAST(@retention_days AS integer)::text
+WHERE site_configs.key = 'agents_chat_retention_days';