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/models.go

@@ -4218,6 +4218,11 @@ type ChatFile struct {
 	Data           []byte    `db:"data" json:"data"`
 }
 
+type ChatFileLink struct {
+	ChatID uuid.UUID `db:"chat_id" json:"chat_id"`
+	FileID uuid.UUID `db:"file_id" json:"file_id"`
+}
+
 type ChatMessage struct {
 	ID                  int64                 `db:"id" json:"id"`
 	ChatID              uuid.UUID             `db:"chat_id" json:"chat_id"`