feat: add file/image attachment support to chat input (#22604)

This change adds support for image attachments to chat via add button
and clipboard paste. Files are stored in a new `chat_files` table and
referenced by ID in message content. File data is resolved from storage
at LLM dispatch time, keeping the message content column small.

Upload validates MIME types via content type or content sniffing against
an allowlist (png, jpeg, gif, webp). The retrieval endpoint serves files
with immutable caching headers. On the frontend, uploads start eagerly
on attach with a background fetch to pre-warm the browser HTTP cache so
the timeline renders instantly after send.

coderd/database/models.go

@@ -3926,6 +3926,16 @@ type ChatDiffStatus struct {
 	GitRemoteOrigin  string         `db:"git_remote_origin" json:"git_remote_origin"`
 }
 
+type ChatFile struct {
+	ID             uuid.UUID `db:"id" json:"id"`
+	OwnerID        uuid.UUID `db:"owner_id" json:"owner_id"`
+	OrganizationID uuid.UUID `db:"organization_id" json:"organization_id"`
+	CreatedAt      time.Time `db:"created_at" json:"created_at"`
+	Name           string    `db:"name" json:"name"`
+	Mimetype       string    `db:"mimetype" json:"mimetype"`
+	Data           []byte    `db:"data" json:"data"`
+}
+
 type ChatMessage struct {
 	ID                  int64                 `db:"id" json:"id"`
 	ChatID              uuid.UUID             `db:"chat_id" json:"chat_id"`