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>

db/imagestore.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"nearle/config"
 	"sort"
 	"strings"
 	"sync"
@@ -33,23 +34,20 @@ var ImageStore *imageStore
 
 // connectImageStore wires up the DigitalOcean Spaces (S3-compatible) client
 // used to resolve catalogue product images. Like the catalogue DB, this must
-// never block or fail app startup — if S3 env vars are absent, image URLs
-// are simply omitted from catalogue responses.
-func connectImageStore() {
-	if getEnv("USE_S3", "") != "true" {
-		fmt.Println("⚠️  S3 not enabled, skipping image store")
+// never block or fail app startup — with USE_S3 unset, image URLs are simply
+// omitted from catalogue responses. (USE_S3=true with a key missing is caught
+// by config.Load before we get here.)
+func connectImageStore(c config.S3Config) {
+	if !c.Enabled {
+		fmt.Println("⚠️  USE_S3 not set, skipping image store")
 		return
 	}
 
-	endpoint := getEnv("S3_ENDPOINT", "")
-	bucket := getEnv("S3_BUCKET", "")
-	accessKey := getEnv("S3_ACCESS_KEY", "")
-	secretKey := getEnv("S3_SECRET_KEY", "")
-	region := getEnv("S3_REGION", "")
-	if endpoint == "" || bucket == "" || accessKey == "" || secretKey == "" {
-		fmt.Println("⚠️  S3 env vars incomplete, skipping image store")
-		return
-	}
+	endpoint := c.Endpoint
+	bucket := c.Bucket
+	accessKey := c.AccessKey
+	secretKey := c.SecretKey
+	region := c.Region
 
 	// S3_ENDPOINT is bucket-qualified (e.g. https://nearle.sgp1.digitaloceanspaces.com).
 	// The SDK's virtual-hosted-style client re-prepends the bucket to whatever