feat: 支持固定类型 Provider OAuth 授权

- 新增 provider_type 字段区分自定义/预置 Provider 类型（claude_code/codex/gemini_cli/antigravity）
- 实现完整 OAuth 2.0 授权流程：start（生成授权 URL + PKCE）、complete（换取 token）、refresh
- 前端 KeyFormDialog 添加 OAuth 授权 UI，支持开始授权、粘贴回调 URL、完成授权、强制刷新
- 请求时自动检测 token 过期并刷新（120s 预留窗口 + Redis 分布式锁防并发）
- 固定类型 Provider 自动创建预置端点并锁定 base_url/custom_path
- 数据库迁移：添加 providers.provider_type，扩展 api_key 列为 TEXT
- 可选依赖 tls-client 用于 Claude token 请求的 TLS 指纹伪装

alembic/versions/20260204_1500_b5c6d7e8f9a0_expand_api_key_column_to_text.py

@@ -0,0 +1,64 @@
+"""Add provider_type and expand api_key column to TEXT
+
+- Add providers.provider_type (String(20), server_default="custom")
+- Change provider_api_keys.api_key from VARCHAR(500) to TEXT (OAuth tokens can be long)
+
+Revision ID: b5c6d7e8f9a0
+Revises: c4e8f9a1b2c3
+Create Date: 2026-02-04 15:00:00.000000
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy import inspect
+
+# revision identifiers, used by Alembic.
+revision: str = "b5c6d7e8f9a0"
+down_revision: Union[str, None] = "c4e8f9a1b2c3"
+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:
+    """检查列是否已存在"""
+    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 providers.provider_type
+    if not column_exists("providers", "provider_type"):
+        op.add_column(
+            "providers",
+            sa.Column("provider_type", sa.String(20), nullable=False, server_default="custom"),
+        )
+
+    # Expand provider_api_keys.api_key from VARCHAR(500) to TEXT
+    op.alter_column(
+        "provider_api_keys",
+        "api_key",
+        type_=sa.Text(),
+        existing_type=sa.String(500),
+        existing_nullable=False,
+    )
+
+
+def downgrade() -> None:
+    # Revert provider_api_keys.api_key from TEXT to VARCHAR(500)
+    # WARNING: Downgrade may fail if any api_key values exceed 500 characters
+    op.alter_column(
+        "provider_api_keys",
+        "api_key",
+        type_=sa.String(500),
+        existing_type=sa.Text(),
+        existing_nullable=False,
+    )
+
+    # Drop providers.provider_type
+    if column_exists("providers", "provider_type"):
+        op.drop_column("providers", "provider_type")