feat: track chat file associations with chat_file_links on chats (#23537)

Needed by #23833

Adds a `chat_file_links` association table to track which files are
associated with each chat.

- `AppendChatFileIDs` query links a file to a chat with deduplication
- `GetChatFileMetadataByIDs` query returns lightweight file metadata by
IDs
- Tool-created files (e.g. `propose_plan`) are linked to the chat after
insert
- User-uploaded files are linked to the chat when the referencing
message is sent
- Single-chat GET endpoint hydrates `files: ChatFileMetadata[]` on the
response

> 🤖 Created by Coder Agents and massaged into shape by a human.

coderd/database/queries/chatfiles.sql

@@ -8,3 +8,13 @@ SELECT * FROM chat_files WHERE id = @id::uuid;
 
 -- name: GetChatFilesByIDs :many
 SELECT * FROM chat_files WHERE id = ANY(@ids::uuid[]);
+
+-- name: GetChatFileMetadataByChatID :many
+-- GetChatFileMetadataByChatID returns lightweight file metadata for
+-- all files linked to a chat. The data column is excluded to avoid
+-- loading file content.
+SELECT cf.id, cf.owner_id, cf.organization_id, cf.name, cf.mimetype, cf.created_at
+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;