fix(db): drop files_user_id_fkey — anonymous users may not exist in users table

The conversations table has no FK on user_id, but files had one, causing
500 errors on file upload when the anonymous user wasn't registered.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/init-db.sql

@@ -1604,7 +1604,7 @@ CREATE TABLE files (
     -- 租户ID
     tenant_id UUID NOT NULL DEFAULT '00000000-0000-0000-0000-000000000001',
     -- 所属用户ID
-    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    user_id UUID NOT NULL,
     -- 关联的对话ID（可选）
     conversation_id UUID REFERENCES conversations(id) ON DELETE SET NULL,
     -- 原始文件名

scripts/migrations/20260207_drop_files_user_fk.sql

@@ -0,0 +1,3 @@
+-- Drop foreign key constraint on files.user_id
+-- Anonymous users may not exist in users table (conversations also has no FK on user_id)
+ALTER TABLE files DROP CONSTRAINT IF EXISTS files_user_id_fkey;