refactor: 实现 ConversionEngine 协议转换引擎，替代旧 protocol 包

引入 Canonical Model 和 ProtocolAdapter 架构，支持 OpenAI/Anthropic 协议间
无缝转换，统一 ProxyHandler 替代分散的 OpenAI/Anthropic Handler，简化
ProviderClient 为协议无关的 HTTP 发送器，Provider 新增 protocol 字段。

backend/migrations/20260401000001_initial_schema.sql

@@ -0,0 +1,33 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS providers (
+    id TEXT PRIMARY KEY,
+    name TEXT NOT NULL,
+    api_key TEXT NOT NULL,
+    base_url TEXT NOT NULL,
+    enabled INTEGER DEFAULT 1,
+    created_at DATETIME,
+    updated_at DATETIME
+);
+
+CREATE TABLE IF NOT EXISTS models (
+    id TEXT PRIMARY KEY,
+    provider_id TEXT NOT NULL,
+    model_name TEXT NOT NULL,
+    enabled INTEGER DEFAULT 1,
+    created_at DATETIME,
+    FOREIGN KEY (provider_id) REFERENCES providers(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS usage_stats (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    provider_id TEXT NOT NULL,
+    model_name TEXT NOT NULL,
+    request_count INTEGER DEFAULT 0,
+    date DATE NOT NULL,
+    UNIQUE(provider_id, model_name, date)
+);
+
+-- +goose Down
+DROP TABLE IF EXISTS usage_stats;
+DROP TABLE IF EXISTS models;
+DROP TABLE IF EXISTS providers;