docs: 完善仓库文档（README / LICENSE / CONTRIBUTING / CHANGELOG / Issue 与 PR 模板）

- README.md: 仓库门面，含特性、API 文档、快速开始、部署、目录结构、协议

- LICENSE: MIT，约束代码

- ASSET_LICENSE.md: CC BY-NC-SA 4.0，约束表情包素材

- CONTRIBUTING.md: 投稿流程、commit 与分支规范、PR 自检清单

- CHANGELOG.md: Keep a Changelog 格式，1.0.0 首发条目

- .gitignore: 补充 macOS/Windows/IDE/Python/日志等忽略项

- .github/ISSUE_TEMPLATE/: bug / feature / question 三个模板

- .github/PULL_REQUEST_TEMPLATE.md: 含素材来源声明与自检清单

LICENSE

@@ -0,0 +1,41 @@
+MIT License
+
+Copyright (c) 2024-2026 shumengya and meme-api contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+================================================================================
+NOTICE — 本仓库采用 双重许可 (Dual Licensing)
+================================================================================
+
+本 LICENSE 文件**仅适用于本仓库中的源代码部分**，包括但不限于：
+  - HTML / CSS / JavaScript 前端代码
+  - Python 脚本（转化.py 等）
+  - Node.js 脚本（scripts/*.mjs）
+  - Cloudflare Pages 配置文件（_headers、_redirects 等）
+  - 文档（README.md、CHANGELOG.md、CONTRIBUTING.md 等 Markdown 文本）
+
+本仓库中**表情包素材**（`public/meme/` 目录下的所有 .webp / .gif 文件，
+以及 `原图/`、`原图-已处理/` 下的所有图片）适用单独的协议，请参阅：
+
+  -> ASSET_LICENSE.md （CC BY-NC-SA 4.0）
+
+简言之：
+  · 代码：MIT，最宽松可用，包括商用
+  · 素材：CC BY-NC-SA 4.0，署名 + 非商用 + 相同方式共享