feat: 添加提供商格式转换优先级保持配置

- 新增 Provider.keep_priority_on_conversion 字段，控制格式转换时是否保持优先级
- 新增全局配置 KEEP_PRIORITY_ON_CONVERSION，可全局启用优先级保持
- 调度器根据配置决定是否降级需要格式转换的候选
- 前端 Provider 表单添加"保持优先级"开关
- 调整 HTTP 读写超时默认值为 3600 秒

alembic/versions/20260128_1200_add_keep_priority_on_conversion.py

@@ -0,0 +1,58 @@
+"""add_keep_priority_on_conversion_to_providers
+
+Revision ID: 364680d1bc99
+Revises: f7c8d9e0a1b2
+Create Date: 2026-01-28 12:00:00+00:00
+
+Changes:
+1. providers 表: 添加 keep_priority_on_conversion 字段
+   - 格式转换时是否保持提供商原优先级
+   - 默认 False：需要格式转换时，候选会被降级到不需要转换的候选之后
+   - 设为 True：即使需要格式转换，也保持原优先级排名
+"""
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy import inspect
+
+
+# revision identifiers, used by Alembic.
+revision = "364680d1bc99"
+down_revision = "f7c8d9e0a1b2"
+branch_labels = None
+depends_on = None
+
+
+def table_exists(table_name: str) -> bool:
+    bind = op.get_bind()
+    inspector = inspect(bind)
+    return table_name in inspector.get_table_names()
+
+
+def column_exists(table_name: str, column_name: str) -> bool:
+    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:
+    # === providers 表: 添加格式转换优先级保持配置 ===
+    if table_exists("providers"):
+        if not column_exists("providers", "keep_priority_on_conversion"):
+            op.add_column(
+                "providers",
+                sa.Column(
+                    "keep_priority_on_conversion",
+                    sa.Boolean(),
+                    nullable=False,
+                    server_default="false",
+                ),
+            )
+
+
+def downgrade() -> None:
+    # === providers 表: 移除格式转换优先级保持配置 ===
+    if table_exists("providers"):
+        if column_exists("providers", "keep_priority_on_conversion"):
+            op.drop_column("providers", "keep_priority_on_conversion")