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/dump.sql

@@ -1190,6 +1190,16 @@ CREATE TABLE chat_diff_statuses (
     git_remote_origin text DEFAULT ''::text NOT NULL
 );
 
+CREATE TABLE chat_files (
+    id uuid DEFAULT gen_random_uuid() NOT NULL,
+    owner_id uuid NOT NULL,
+    organization_id uuid NOT NULL,
+    created_at timestamp with time zone DEFAULT now() NOT NULL,
+    name text DEFAULT ''::text NOT NULL,
+    mimetype text NOT NULL,
+    data bytea NOT NULL
+);
+
 CREATE TABLE chat_messages (
     id bigint NOT NULL,
     chat_id uuid NOT NULL,
@@ -3140,6 +3150,9 @@ ALTER TABLE ONLY boundary_usage_stats
 ALTER TABLE ONLY chat_diff_statuses
     ADD CONSTRAINT chat_diff_statuses_pkey PRIMARY KEY (chat_id);
 
+ALTER TABLE ONLY chat_files
+    ADD CONSTRAINT chat_files_pkey PRIMARY KEY (id);
+
 ALTER TABLE ONLY chat_messages
     ADD CONSTRAINT chat_messages_pkey PRIMARY KEY (id);
 
@@ -3495,6 +3508,10 @@ CREATE INDEX idx_audit_logs_time_desc ON audit_logs USING btree ("time" DESC);
 
 CREATE INDEX idx_chat_diff_statuses_stale_at ON chat_diff_statuses USING btree (stale_at);
 
+CREATE INDEX idx_chat_files_org ON chat_files USING btree (organization_id);
+
+CREATE INDEX idx_chat_files_owner ON chat_files USING btree (owner_id);
+
 CREATE INDEX idx_chat_messages_chat ON chat_messages USING btree (chat_id);
 
 CREATE INDEX idx_chat_messages_chat_created ON chat_messages USING btree (chat_id, created_at);
@@ -3774,6 +3791,12 @@ ALTER TABLE ONLY api_keys
 ALTER TABLE ONLY chat_diff_statuses
     ADD CONSTRAINT chat_diff_statuses_chat_id_fkey FOREIGN KEY (chat_id) REFERENCES chats(id) ON DELETE CASCADE;
 
+ALTER TABLE ONLY chat_files
+    ADD CONSTRAINT chat_files_organization_id_fkey FOREIGN KEY (organization_id) REFERENCES organizations(id) ON DELETE CASCADE;
+
+ALTER TABLE ONLY chat_files
+    ADD CONSTRAINT chat_files_owner_id_fkey FOREIGN KEY (owner_id) REFERENCES users(id) ON DELETE CASCADE;
+
 ALTER TABLE ONLY chat_messages
     ADD CONSTRAINT chat_messages_chat_id_fkey FOREIGN KEY (chat_id) REFERENCES chats(id) ON DELETE CASCADE;