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>

config.example.yaml

@@ -1,6 +1,6 @@
 # Minstrel example configuration.
 # Every field is optional; defaults shown below match Default() in internal/config.
-# Each setting can be overridden at runtime via SMARTMUSIC_* environment variables
+# Each setting can be overridden at runtime via MINSTREL_* environment variables
 # (see internal/config/config.go for the full list).
 
 server:
@@ -11,7 +11,7 @@ storage:
   # collages under <data_dir>/playlist_covers/). Default: "./data" relative
   # to the working directory. Production deployments should set an absolute
   # path on persistent storage.
-  # Env: SMARTMUSIC_STORAGE_DATA_DIR
+  # Env: MINSTREL_STORAGE_DATA_DIR
   data_dir: "./data"
 
 database:
@@ -24,25 +24,26 @@ log:
 
 auth:
   # One-shot admin bootstrap. Applied only when the users table is empty.
-  # Set username to enable; leave password blank to have the server generate
-  # one and log it to stderr on first start.
-  # Env: SMARTMUSIC_AUTH_ADMIN_USERNAME, SMARTMUSIC_AUTH_ADMIN_PASSWORD
+  # Username defaults to "admin" when unset — you can omit this section
+  # entirely for a working bootstrap. Leave password blank to have the server
+  # generate one and log it to stderr on first start.
+  # Env: MINSTREL_AUTH_ADMIN_USERNAME, MINSTREL_AUTH_ADMIN_PASSWORD
   admin_bootstrap:
-    username: ""
+    username: "admin"
     password: ""
 
 library:
   # Filesystem roots to scan for music. Each path is walked recursively.
-  # Env: SMARTMUSIC_LIBRARY_SCAN_PATHS (colon-separated, PATH-style)
+  # Env: MINSTREL_LIBRARY_SCAN_PATHS (colon-separated, PATH-style)
   scan_paths: []
-  # Kick off a scan on server startup. Env: SMARTMUSIC_LIBRARY_SCAN_ON_STARTUP
+  # Kick off a scan on server startup. Env: MINSTREL_LIBRARY_SCAN_ON_STARTUP
   scan_on_startup: false
 
 # Per-instance branding. Operators can rename their instance ("Family
 # Jukebox", "Office Music", etc.) and override the OG share-preview
 # description. Both fields default to sensible Minstrel values when
 # unset; you only need this section to override.
-# Env: SMARTMUSIC_BRANDING_APP_NAME, SMARTMUSIC_BRANDING_DESCRIPTION
+# Env: MINSTREL_BRANDING_APP_NAME, MINSTREL_BRANDING_DESCRIPTION
 branding:
   app_name: "Minstrel"
   description: "Self-hosted music server with Subsonic compatibility, smart shuffle, and Lidarr integration."
@@ -51,7 +52,7 @@ subsonic:
   # Gate for the legacy `p=` plaintext password parameter on /rest/*.
   # Leave disabled unless a specific client requires it — Minstrel prefers
   # apiKey (OpenSubsonic) and falls back to u+t+s token auth.
-  # Env: SMARTMUSIC_SUBSONIC_ALLOW_PLAINTEXT_PASSWORD
+  # Env: MINSTREL_SUBSONIC_ALLOW_PLAINTEXT_PASSWORD
   allow_plaintext_password: false
 
 events: