chore(config): rename SMARTMUSIC_ env prefix to MINSTREL_; default admin user

The legacy SMARTMUSIC_ prefix dates from when the project was specced
as "smart-music" before being renamed to Minstrel. Hard cutover to
MINSTREL_* across config.go, tests, config.example.yaml, README, and
docker-compose. Also renames SMARTMUSIC_CONFIG in cmd/minstrel/main.go
which was missed in the original audit.

Also seeds Auth.AdminBootstrap.Username = "admin" in Default() so
MINSTREL_DATABASE_URL is the only env var required for a fresh prod
deployment — the bootstrap auto-generates a password and prints it
to stderr. Drops the now-redundant MINSTREL_AUTH_ADMIN_USERNAME=admin
line from the README quickstart and docker-compose.

M1 follow-up from the #363 final review: guards
MINSTREL_BRANDING_APP_NAME and _DESCRIPTION against empty-string
overrides — empty would have shipped <title></title> to share-preview
crawlers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -34,12 +34,11 @@ services:
       postgres:
         condition: service_healthy
     environment:
-      SMARTMUSIC_DATABASE_URL: postgres://minstrel:minstrel@postgres:5432/minstrel?sslmode=disable
-      SMARTMUSIC_LOG_FORMAT: text
-      SMARTMUSIC_LIBRARY_SCAN_PATHS: /music
-      SMARTMUSIC_LIBRARY_SCAN_ON_STARTUP: "true"
-      SMARTMUSIC_AUTH_ADMIN_USERNAME: admin
-      # SMARTMUSIC_AUTH_ADMIN_PASSWORD left unset on purpose — bootstrap
+      MINSTREL_DATABASE_URL: postgres://minstrel:minstrel@postgres:5432/minstrel?sslmode=disable
+      MINSTREL_LOG_FORMAT: text
+      MINSTREL_LIBRARY_SCAN_PATHS: /music
+      MINSTREL_LIBRARY_SCAN_ON_STARTUP: "true"
+      # MINSTREL_AUTH_ADMIN_PASSWORD left unset on purpose — bootstrap
       # generates one and prints it to stderr on first boot. Watch the logs
       # of the minstrel service the first time you bring the stack up.
     networks: