refactor: 统一配置文件系统

- 补全 backend/config.yaml 所有非敏感配置项并添加中文注释
- 重写 config.go：Load(workDir) 显式传参，BindEnv 绑定敏感字段，删除 AutomaticEnv
- setDefaults 默认值与 config.yaml 保持一致（mimo/dashscope）
- backend/.env.example 重写为纯敏感信息模板
- .env 固定在 /opt/camtalk/.env，docker-compose 通过绝对路径加载
- deploy.sh 统一使用 --env-file，移除硬编码 IP
- deploy.yml 删除 CI 写入 .env 的步骤
- Dockerfile 移除 COPY config.yaml
- 修复 deploy.yml 中 POSTGRES_PASSWORD 的 &{{ 拼写错误

backend/go.mod

@@ -4,13 +4,16 @@ go 1.25.0
 
 require (
 	github.com/gin-gonic/gin v1.10.0
+	github.com/golang-jwt/jwt/v5 v5.3.1
 	github.com/google/uuid v1.6.0
 	github.com/gorilla/websocket v1.5.3
 	github.com/jackc/pgx/v5 v5.10.0
+	github.com/joho/godotenv v1.5.1
 	github.com/redis/go-redis/v9 v9.20.1
 	github.com/spf13/viper v1.21.0
 	github.com/stretchr/testify v1.11.1
 	go.uber.org/zap v1.28.0
+	golang.org/x/crypto v0.23.0
 )
 
 require (
@@ -28,7 +31,6 @@ require (
 	github.com/go-playground/validator/v10 v10.20.0 // indirect
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/goccy/go-json v0.10.2 // indirect
-	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
@@ -53,7 +55,6 @@ require (
 	go.uber.org/multierr v1.10.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/arch v0.8.0 // indirect
-	golang.org/x/crypto v0.23.0 // indirect
 	golang.org/x/net v0.25.0 // indirect
 	golang.org/x/sync v0.17.0 // indirect
 	golang.org/x/sys v0.30.0 // indirect