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>

messaging/livehub.go

@@ -119,7 +119,15 @@ func StartLiveHub() *LiveHub {
 		SetCleanSession(true). // No queued backlog on reconnect; stale nudges are noise.
 		SetOrderMatters(false)
 
-	if user := strings.TrimSpace(os.Getenv("MQTT_USERNAME")); user != "" {
+	// MQTT_USER is the name the ingest consumer and the env files use. This
+	// read MQTT_USERNAME for a while, so the live stream connected to the
+	// production broker with no credentials at all; MQTT_USERNAME is still
+	// honoured for any deployment that set it.
+	user := strings.TrimSpace(os.Getenv("MQTT_USER"))
+	if user == "" {
+		user = strings.TrimSpace(os.Getenv("MQTT_USERNAME"))
+	}
+	if user != "" {
 		opts.SetUsername(user)
 		opts.SetPassword(os.Getenv("MQTT_PASSWORD"))
 	}