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>

Dockerfile

@@ -14,6 +14,13 @@ WORKDIR /app
 COPY --from=builder /app/server /app
 COPY nearle-gear-firebase-adminsdk-l9oha-23ca3b3609.json .
 
+# No `.env.*` is copied in (see .dockerignore), so this only decides which
+# rules config.Load applies: production insists on a signing secret and never
+# falls back to localhost values. Every real value comes from the platform's
+# environment settings.
+ENV APP_ENV=production
+
+# Must match APP_PORT in the platform's environment (1009 in production).
 EXPOSE 1009
 
 CMD ["/app/server"]