feat: zero-config first boot and pluggable storage backend

Two changes that drop the friction in self-hosting RefBoard so the install
story becomes "docker compose up, open the URL".

1. JWT_SECRET is now optional. On first boot the backend generates a
   64-byte random secret and persists it in the existing settings table.
   process.env.JWT_SECRET still wins when set, so ops setups that manage
   secrets out-of-band are unaffected. The prod-throws-without-env guard
   is gone (auto-generation is a strictly safer default than the previous
   hardcoded dev fallback).

2. STORAGE_BACKEND=fs|minio picks between MinIO (default, unchanged) and
   a new local-filesystem adapter. The FS adapter exposes a fake minioClient
   that mirrors the methods RefBoard calls (statObject, getObject,
   getPartialObject, listObjectsV2, putObject, removeObject(s), bucketExists,
   makeBucket), so consumers swap require('./minio') for require('./storage')
   and nothing else changes. Sidecar .mime files hold Content-Type so the
   range-aware media proxy still serves the right response headers.

examples/compose/minimal-fs.yml is the single-container variant that uses
the FS adapter. The default docker-compose.yml still spins up MinIO.

README quick-start collapses to one block (cp .env, docker compose pull,
docker compose up -d). The first user you register on the Login screen is
auto-promoted to admin, same as before.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/routes/collections.js

@@ -14,7 +14,7 @@ const {
   getCollectionBoards,
   getUserByEmail,
 } = require('../db');
-const { deleteBoardImages: deleteBoardMinioImages } = require('../minio');
+const { deleteBoardImages: deleteBoardMinioImages } = require('../storage');
 
 const router = Router();