refactor: 移除 Python 后端源码，全面迁移至 Rust gateway 架构

- 删除全部 Python 源码 (src/) 及 Alembic 迁移脚本，归档至 _deprecated_py_src/
- 重构 Rust gateway ai_pipeline: 拆分 planner/finalize 模块，新增 contracts/adaptation 层
- 重组 handlers 模块为 admin/public/proxy/internal/shared 子模块结构
- 新增 executor 模块，引入 Rust 原生数据库迁移 (aether-data/migrations)
- 简化 CI/Docker 构建流程，移除 base image 二级构建，统一为单一 app image
- 移除 Python 相关基础设施文件 (entrypoint.sh, gunicorn_conf.py, Dockerfile.base)

_deprecated_py_src/api/handlers/gemini/image_gen.py

@@ -0,0 +1,41 @@
+"""
+Gemini 图像生成模型请求适配
+
+- 图像生成模型不支持 tools / system_instruction，需要移除
+- responseModalities / responseMimeType 与 imageConfig 冲突，需要移除
+"""
+
+from typing import Any
+
+from src.core.video_utils import is_image_gen_model
+
+__all__ = ["is_image_gen_model", "adapt_request_for_image_gen"]
+
+
+def adapt_request_for_image_gen(body: dict[str, Any]) -> dict[str, Any]:
+    """为图像生成模型清理不兼容字段"""
+    # 移除图像生成不支持的顶层字段
+    for key in ("tools", "tool_config", "toolConfig", "system_instruction", "systemInstruction"):
+        if key in body:
+            body.pop(key)
+
+    # 处理 generationConfig
+    gc_key = "generationConfig" if "generationConfig" in body else "generation_config"
+    gc = body.get(gc_key)
+    if not isinstance(gc, dict):
+        gc = {}
+        body[gc_key] = gc
+
+    # 移除与图像生成冲突的字段
+    for key in (
+        "responseMimeType",
+        "response_mime_type",
+        "responseModalities",
+        "response_modalities",
+    ):
+        gc.pop(key, None)
+
+    # 设置输出模态
+    gc["responseModalities"] = ["TEXT", "IMAGE"]
+
+    return body