完善部分平台治理功能，及修改看板缺陷

backend/app/db/sql/004_permissions.sql

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+-- ============================================================
+-- 权限体系扩展：GPU 分配表 + 资源所有权字段
+-- ============================================================
+
+-- GPU 分配表：管理员指定哪些用户可以使用哪些 GPU 卡
+CREATE TABLE IF NOT EXISTS gpu_assignments (
+    id          TEXT PRIMARY KEY,
+    node_id     TEXT NOT NULL REFERENCES compute_nodes(id) ON DELETE CASCADE,
+    gpu_index   INTEGER NOT NULL,
+    user_id     TEXT NOT NULL,
+    assigned_by TEXT,
+    assigned_at TEXT NOT NULL,
+    UNIQUE (node_id, gpu_index, user_id)
+);
+CREATE INDEX IF NOT EXISTS idx_gpu_assignments_user ON gpu_assignments(user_id);
+CREATE INDEX IF NOT EXISTS idx_gpu_assignments_gpu ON gpu_assignments(node_id, gpu_index);
+
+-- 资源所有权字段：用户创建的数据集/模型/训练产物/评测任务
+ALTER TABLE datasets ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE models ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE trained_models ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE eval_tasks ADD COLUMN IF NOT EXISTS created_by TEXT;