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/migrations/000462_chat_file_links.down.sql

@@ -0,0 +1,9 @@
+ALTER TABLE chats ADD COLUMN file_ids uuid[] DEFAULT '{}'::uuid[] NOT NULL;
+
+UPDATE chats SET file_ids = (
+    SELECT COALESCE(array_agg(cfl.file_id), '{}')
+    FROM chat_file_links cfl
+    WHERE cfl.chat_id = chats.id
+);
+
+DROP TABLE chat_file_links;

coderd/database/migrations/000462_chat_file_links.up.sql

@@ -0,0 +1,17 @@
+CREATE TABLE chat_file_links (
+    chat_id uuid NOT NULL,
+    file_id uuid NOT NULL,
+    UNIQUE (chat_id, file_id)
+);
+
+CREATE INDEX idx_chat_file_links_chat_id ON chat_file_links (chat_id);
+
+ALTER TABLE chat_file_links
+    ADD CONSTRAINT chat_file_links_chat_id_fkey
+    FOREIGN KEY (chat_id) REFERENCES chats(id) ON DELETE CASCADE;
+
+ALTER TABLE chat_file_links
+    ADD CONSTRAINT chat_file_links_file_id_fkey
+    FOREIGN KEY (file_id) REFERENCES chat_files(id) ON DELETE CASCADE;
+
+ALTER TABLE chats DROP COLUMN IF EXISTS file_ids;

coderd/database/migrations/testdata/fixtures/000462_chat_file_links.up.sql

@@ -0,0 +1,5 @@
+INSERT INTO chat_file_links (chat_id, file_id)
+VALUES (
+    '72c0438a-18eb-4688-ab80-e4c6a126ef96',
+    '00000000-0000-0000-0000-000000000099'
+);