feat(session): support opt-in Secure session cookies

- add SESSION_COOKIE_SECURE / SESSION_COOKIE_TRUSTED_URL env vars with
  startup validation: enabling Secure requires at least one trusted
  HTTPS entry URL
- wire common.SessionCookieSecure into the session cookie store instead
  of a hardcoded Secure=false
- print a startup warning when Secure session cookies are disabled
- document the new settings in .env.example and docker-compose files

Secure stays off by default because many deployments front new-api with
plain-HTTP reverse proxies, where a hardcoded Secure default would break
logins entirely; enabling it safely depends on the deployment's TLS
setup, so it ships as an opt-in deployment-hardening flag.

docker-compose.yml

@@ -39,6 +39,8 @@ services:
 #      - STREAMING_TIMEOUT=300  # 流模式无响应超时时间，单位秒，默认120秒，如果出现空补全可以尝试改为更大值 （Streaming timeout in seconds, default is 120s. Increase if experiencing empty completions）
 #      - RELAY_IDLE_CONN_TIMEOUT=90  # Relay HTTP 客户端空闲连接超时时间，单位秒，默认跟随 Go 标准库，设置为0表示不限制 (Relay HTTP client idle keep-alive timeout in seconds, defaults to Go standard library; set 0 to disable)
 #      - SESSION_SECRET=random_string  # 多机部署时设置，必须修改这个随机字符串！！ （multi-node deployment, set this to a random string!!!!!!!）
+#      - SESSION_COOKIE_SECURE=true  # 启用 Secure session cookie，必须同时配置 SESSION_COOKIE_TRUSTED_URL (Enable Secure session cookies; requires SESSION_COOKIE_TRUSTED_URL)
+#      - SESSION_COOKIE_TRUSTED_URL=https://example.com,https://admin.example.com  # 可信 HTTPS 入口地址，多个用英文逗号分隔 (Trusted HTTPS entry URLs, comma-separated)
 #      - SYNC_FREQUENCY=60  # Uncomment if regular database syncing is needed
 #      - GOOGLE_ANALYTICS_ID=G-XXXXXXXXXX  # Google Analytics 的测量 ID (Google Analytics Measurement ID)
 #      - UMAMI_WEBSITE_ID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx  # Umami 网站 ID (Umami Website ID)