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>

go.mod

@@ -12,6 +12,7 @@ require (
 	github.com/gofiber/fiber v1.14.6
 	github.com/joho/godotenv v1.5.1
 	github.com/redis/go-redis/v9 v9.18.0
+	github.com/valyala/fasthttp v1.50.0
 	golang.org/x/oauth2 v0.12.0
 	google.golang.org/api v0.143.0
 	gorm.io/driver/postgres v1.6.0
@@ -42,8 +43,8 @@ require (
 	github.com/aws/aws-sdk-go-v2/service/sts v1.44.1 // indirect
 	github.com/aws/smithy-go v1.27.3 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
-	github.com/fsnotify/fsnotify v1.7.0 // indirect
 	github.com/gofiber/utils v0.0.10 // indirect
 	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
 	github.com/golang/protobuf v1.5.3 // indirect
@@ -54,7 +55,6 @@ require (
 	github.com/googleapis/gax-go/v2 v2.12.0 // indirect
 	github.com/gorilla/schema v1.1.0 // indirect
 	github.com/gorilla/websocket v1.5.3 // indirect
-	github.com/hashicorp/hcl v1.0.0 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/pgx/v5 v5.6.0 // indirect
@@ -62,28 +62,17 @@ require (
 	github.com/jinzhu/inflection v1.0.0 // indirect
 	github.com/jinzhu/now v1.1.5 // indirect
 	github.com/klauspost/compress v1.19.0 // indirect
-	github.com/magiconair/properties v1.8.7 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/mattn/go-runewidth v0.0.15 // indirect
-	github.com/mitchellh/mapstructure v1.5.0 // indirect
+	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/rivo/uniseg v0.4.4 // indirect
-	github.com/rogpeppe/go-internal v1.11.0 // indirect
-	github.com/sagikazarmark/locafero v0.3.0 // indirect
-	github.com/sagikazarmark/slog-shim v0.1.0 // indirect
-	github.com/sourcegraph/conc v0.3.0 // indirect
-	github.com/spf13/afero v1.10.0 // indirect
-	github.com/spf13/cast v1.5.1 // indirect
-	github.com/spf13/pflag v1.0.5 // indirect
-	github.com/subosito/gotenv v1.6.0 // indirect
+	github.com/stretchr/testify v1.8.4 // indirect
 	github.com/valyala/bytebufferpool v1.0.0 // indirect
-	github.com/valyala/fasthttp v1.50.0 // indirect
 	github.com/valyala/tcplisten v1.0.0 // indirect
 	go.opencensus.io v0.24.0 // indirect
 	go.uber.org/atomic v1.11.0 // indirect
-	go.uber.org/multierr v1.11.0 // indirect
 	golang.org/x/crypto v0.31.0 // indirect
-	golang.org/x/exp v0.0.0-20231006140011-7918f672742d // indirect
 	golang.org/x/net v0.33.0 // indirect
 	golang.org/x/sync v0.10.0 // indirect
 	golang.org/x/time v0.3.0 // indirect
@@ -94,15 +83,11 @@ require (
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20230920204549-e6e6cdab5c13 // indirect
 	google.golang.org/grpc v1.58.2 // indirect
 	google.golang.org/protobuf v1.31.0 // indirect
-	gopkg.in/ini.v1 v1.67.0 // indirect
 )
 
 require (
 	github.com/gofiber/fiber/v2 v2.50.0
 	github.com/jinzhu/copier v0.4.0
-	github.com/pelletier/go-toml/v2 v2.1.0 // indirect
-	github.com/spf13/viper v1.17.0
 	golang.org/x/sys v0.28.0 // indirect
 	golang.org/x/text v0.21.0 // indirect
-	gopkg.in/yaml.v3 v3.0.1 // indirect
 )