feat: 添加请求体规则功能和 UI 优化

- 添加 body_rules 支持（set/drop/rename 操作）
- 后端 apply_body_rules 函数处理请求体修改
- 保护 model/stream 字段不被修改
- 前端 UI 重构：合并请求头/请求体规则为统一区域
- 使用左边框颜色区分规则类型（H=主题色，B=灰色）

Closes #139

Co-authored-by: CC <81609553+hemo94931@users.noreply.github.com>

alembic/versions/20260203_1500_add_video_duration_seconds.py

@@ -1,43 +0,0 @@
-"""add video_duration_seconds to video_tasks
-
-Revision ID: b3c4d5e6f7a8
-Revises: a2f1b3c4d5e6
-Create Date: 2026-02-03 15:00:00.000000
-
-"""
-
-from typing import Sequence, Union
-
-from alembic import op
-import sqlalchemy as sa
-from sqlalchemy import inspect
-
-
-# revision identifiers, used by Alembic.
-revision: str = "b3c4d5e6f7a8"
-down_revision: Union[str, None] = "a2f1b3c4d5e6"
-branch_labels: Union[str, Sequence[str], None] = None
-depends_on: Union[str, Sequence[str], None] = None
-
-
-def _column_exists(table_name: str, column_name: str) -> bool:
-    """Check if a column exists in a table."""
-    bind = op.get_bind()
-    inspector = inspect(bind)
-    columns = [col["name"] for col in inspector.get_columns(table_name)]
-    return column_name in columns
-
-
-def upgrade() -> None:
-    """Add video_duration_seconds column to video_tasks table."""
-    if not _column_exists("video_tasks", "video_duration_seconds"):
-        op.add_column(
-            "video_tasks",
-            sa.Column("video_duration_seconds", sa.Float(), nullable=True),
-        )
-
-
-def downgrade() -> None:
-    """Remove video_duration_seconds column from video_tasks table."""
-    if _column_exists("video_tasks", "video_duration_seconds"):
-        op.drop_column("video_tasks", "video_duration_seconds")