Load .env.<APP_ENV>, validate config at boot, keep secrets out of the image

`main.go` only ever loaded `.env`; the `APP_ENV` switch described in
`.env.local` / `.env.production` did not exist, and a missing variable
surfaced one restart at a time as a log.Fatalf inside db.Connect.

config.Load now picks `.env.<APP_ENV>` (default local) then `.env`, with
real environment winning, reads every setting into one typed Config and
reports everything missing in one message. Production insists on a POS
signing secret; local warns when DB_HOST is not a local address. db,
redis and the image store take the Config instead of reading env
themselves.

Also:
- livehub read MQTT_USERNAME while everything else uses MQTT_USER, so the
  console stream connected to the broker unauthenticated. Both accepted.
- .dockerignore: `COPY . .` was baking .env.production into the image.
  Dockerfile sets APP_ENV=production.
- Drop utils/config.go (dead viper loader) and create_table.go (unused,
  hardcoded production DSN); go mod tidy removes viper.
- .env.example lists every variable the code reads; docs/ENVIRONMENT.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

utils/config.go

@@ -1,51 +0,0 @@
-package utils
-
-import (
-	"fmt"
-	"os"
-
-	"github.com/spf13/viper"
-)
-
-func DevConfig() (Port, DBname, Password, Username, Host, key, secret string) {
-
-	viper.SetConfigName("config") 
-	viper.SetConfigType("yaml")
-	viper.AddConfigPath(".")
-
-	err := viper.ReadInConfig()
-	if err != nil {
-		fmt.Println("fatal error config file: default \n", err)
-		os.Exit(1)
-	}
-
-	dbname := viper.GetString("DEV.DATABASE_NAME")
-	dbpassword := viper.GetString("DEV.DATABASE_PASSWORD")
-	dbusername := viper.GetString("DEV.DATABASE_USERNAME")
-	dbport := viper.GetString("DEV.DATABASE_PORT")
-	dbhost := viper.GetString("DEV.DATABASE_SERVER_HOST")
-	contextkey := viper.GetString("DEV.USER_CONTEXT_KEY")
-	jwtkey := viper.GetString("DEV.JWT_SECRET_KEY")
-	return dbport, dbname, dbpassword, dbusername, dbhost, contextkey, jwtkey
-}
-
-func LiveConfig() (Port, DBname, Password, Username, Host, key, secret string) {
-
-	viper.SetConfigName("config") 
-	viper.SetConfigType("yaml")
-	viper.AddConfigPath(".")
-
-	err := viper.ReadInConfig()
-	if err != nil {
-		fmt.Println("fatal error config file: default \n", err)
-		os.Exit(1)
-	}
-	dbname := viper.GetString("APP.DATABASE_NAME")
-	dbpassword := viper.GetString("APP.DATABASE_PASSWORD")
-	dbusername := viper.GetString("APP.DATABASE_USERNAME")
-	dbport := viper.GetString("APP.DATABASE_PORT")
-	dbhost := viper.GetString("APP.DATABASE_SERVER_HOST")
-	contextkey := viper.GetString("APP.USER_CONTEXT_KEY")
-	jwtkey := viper.GetString("APP.JWT_SECRET_KEY")
-	return dbport, dbname, dbpassword, dbusername, dbhost, contextkey, jwtkey
-}