refactor(server): remove bootstrap admin path

The bootstrap-admin-from-env-vars flow was a holdover from before
self-registration could create the first admin. Now that handleRegister
+ CreateUserFirstAdminRace promotes the first user to register on an
empty users table (verified shipped in v2026.05.08.2 / #376), the
bootstrap path is just a second source of truth that confuses operators
(and leaves an "admin" account in the DB that nobody asked for).

Removes:
- internal/auth/bootstrap.go + its test
- The auth.Bootstrap call from cmd/minstrel/main.go
- AuthConfig + AdminBootstrapConfig structs from config
- MINSTREL_AUTH_ADMIN_USERNAME / _PASSWORD env reads + their test
- The auth: block from config.example.yaml
- Bootstrap-related comments in docker-compose.yml + README.md

The README quickstart now points operators at /register on first start
instead of "watch the logs for a one-time password."

Existing instances keep their bootstrap-admin row in the DB; operators
who want it gone can register a new admin via /register, promote them
in /admin/users, then delete the old bootstrap user (last-admin guard
will require the new admin to be promoted first). No migration needed.

Recovery story for forgotten admin passwords now hinges on Fable #321
(admin password reset CLI) — currently the only path back in if no
other admin exists.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -39,9 +39,6 @@ services:
       MINSTREL_LIBRARY_SCAN_PATHS: /music
       MINSTREL_LIBRARY_SCAN_ON_STARTUP: "true"
       MINSTREL_STORAGE_DATA_DIR: /app/data
-      # 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:
       - minstrel
     ports: