feat: ModelRouter — unified OpenAI-compatible multi-source LLM gateway

- Lua adapters per upstream (transform_request/response/stream_chunk, build_headers signing hooks)
- AUTO priority routing with per-model kind (chat/image), explicit source/model routing
- Per-source concurrency caps with queueing, exponential backoff, AUTO failover
- OpenAI-compatible API: chat completions, SSE streaming, image generations, models
- Gateway key auth, web UI for adapter/source management, runtime persistence
- e2e test running the real binary against mocked upstreams

config.example.yaml

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+# llmsproxy — 统一 OpenAI 兼容网关配置
+
+# 网关监听地址（默认 :8080，建议绑内网/回环）
+listen: 127.0.0.1:8080
+
+# 客户端访问本网关所需的 API Key（Bearer）。留空数组 = 不鉴权（仅内网）。
+gateway_keys:
+  - sk-gw-local-0001
+
+# 默认模型选择：具体模型 id 或 AUTO（按各源模型的 priority 自动选最高可用源）
+default_model: AUTO
+
+# Lua 适配器目录（默认 adapters/，首次启动自动写入内置适配器）
+adapter_dir: adapters
+
+# 运行时持久化文件（WebUI 新增/编辑的源会写入此文件，重启后仍生效）
+runtime_file: runtime.json
+
+# 全局并发上限（0 = 不限）
+max_concurrent: 0
+
+# ---- 上游 LLM 源列表 ----
+# 每个源对应一个 Lua 适配器（adapter）；同一个适配器可被多个源复用。
+sources:
+  - name: deepseek
+    base_url: https://api.deepseek.com
+    api_key: sk-your-deepseek-key
+    adapter: deepseek
+    max_concurrent: 8
+    models:
+      - id: deepseek-v4-flash
+        priority: 100        # 数字越大越优先被 AUTO 选中
+        kind: chat
+      - id: deepseek-reasoner
+        priority: 60
+        kind: chat
+        meta: { thinking: true }
+
+  - name: ollama
+    base_url: http://127.0.0.1:11434
+    api_key: ""
+    adapter: ollama
+    endpoint: /api/chat
+    models:
+      - id: llama3
+        priority: 50
+        kind: chat
+
+  # 演示：KimiCode 校验调用方 app，通过 Lua build_headers 钩子做签名
+  - name: kimicode
+    base_url: https://api.moonshot.cn
+    api_key: sk-your-kimi-key
+    adapter: kimicode
+    models:
+      - id: kimi-k2
+        priority: 90
+        kind: chat
+    meta:
+      app_id: your-app-id
+      app_secret: your-app-secret
+      app_agent: code-agent
+      api_key: sk-your-kimi-key
+
+  # 生图示例：OpenAI 兼容生图源（flux/dall-e 等）
+  - name: imagegen
+    base_url: https://api.example.com
+    api_key: sk-image
+    adapter: openai
+    models:
+      - id: flux-1
+        priority: 80
+        kind: image