feat: 移植截图中心后端（索引/分组/编辑保存）

packages/app-lib/migrations/20261002000000_screenshot-center.sql

@@ -0,0 +1,38 @@
+-- Screenshot center: unified index of per-instance screenshots plus
+-- user-defined groups. Ported from the Axolotl implementation, trimmed of
+-- the instance-sync / game-options tables that Starlight does not use.
+
+CREATE TABLE screenshots (
+	id TEXT NOT NULL PRIMARY KEY,
+	instance_id TEXT NOT NULL,
+	file_name TEXT NOT NULL,
+	content_hash TEXT NOT NULL,
+	file_size INTEGER NOT NULL,
+	modified_at INTEGER NOT NULL,
+	created_at INTEGER NOT NULL,
+	editor_state TEXT,
+	UNIQUE (instance_id, file_name),
+	FOREIGN KEY (instance_id) REFERENCES instances(id) ON DELETE CASCADE
+);
+
+CREATE INDEX screenshots_instance_id ON screenshots(instance_id);
+CREATE INDEX screenshots_instance_hash ON screenshots(instance_id, content_hash, file_size);
+
+CREATE TABLE screenshot_groups (
+	id TEXT NOT NULL PRIMARY KEY,
+	name TEXT NOT NULL,
+	display_order INTEGER NOT NULL DEFAULT 0,
+	CHECK (length(trim(name)) > 0)
+);
+
+CREATE INDEX screenshot_groups_display_order ON screenshot_groups(display_order);
+
+CREATE TABLE screenshot_group_memberships (
+	screenshot_id TEXT NOT NULL PRIMARY KEY,
+	group_id TEXT NOT NULL,
+	FOREIGN KEY (screenshot_id) REFERENCES screenshots(id) ON DELETE CASCADE,
+	FOREIGN KEY (group_id) REFERENCES screenshot_groups(id) ON DELETE CASCADE
+);
+
+CREATE INDEX screenshot_group_memberships_group_id ON screenshot_group_memberships(group_id);
+ALTER TABLE settings ADD COLUMN show_screenshots_tab_in_instances INTEGER NOT NULL DEFAULT FALSE CHECK (show_screenshots_tab_in_instances IN (0, 1));