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/unique_constraint.go

@@ -15,6 +15,7 @@ const (
 	UniqueAuditLogsPkey                                       UniqueConstraint = "audit_logs_pkey"                                                 // ALTER TABLE ONLY audit_logs ADD CONSTRAINT audit_logs_pkey PRIMARY KEY (id);
 	UniqueBoundaryUsageStatsPkey                              UniqueConstraint = "boundary_usage_stats_pkey"                                       // ALTER TABLE ONLY boundary_usage_stats ADD CONSTRAINT boundary_usage_stats_pkey PRIMARY KEY (replica_id);
 	UniqueChatDiffStatusesPkey                                UniqueConstraint = "chat_diff_statuses_pkey"                                         // ALTER TABLE ONLY chat_diff_statuses ADD CONSTRAINT chat_diff_statuses_pkey PRIMARY KEY (chat_id);
+	UniqueChatFilesPkey                                       UniqueConstraint = "chat_files_pkey"                                                 // ALTER TABLE ONLY chat_files ADD CONSTRAINT chat_files_pkey PRIMARY KEY (id);
 	UniqueChatMessagesPkey                                    UniqueConstraint = "chat_messages_pkey"                                              // ALTER TABLE ONLY chat_messages ADD CONSTRAINT chat_messages_pkey PRIMARY KEY (id);
 	UniqueChatModelConfigsPkey                                UniqueConstraint = "chat_model_configs_pkey"                                         // ALTER TABLE ONLY chat_model_configs ADD CONSTRAINT chat_model_configs_pkey PRIMARY KEY (id);
 	UniqueChatProvidersPkey                                   UniqueConstraint = "chat_providers_pkey"                                             // ALTER TABLE ONLY chat_providers ADD CONSTRAINT chat_providers_pkey PRIMARY KEY (id);