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/storage.js

@@ -0,0 +1,18 @@
+// Storage backend router.
+//
+// STORAGE_BACKEND=fs    → local filesystem (single-container deploys, native
+//                          installs, designers who don't want to run MinIO)
+// STORAGE_BACKEND=minio → MinIO / any S3-compatible store (default)
+//
+// Both modules export the same shape (minioClient + helper functions), so
+// consumers `require('./storage')` and never touch the backend choice
+// directly.
+
+const backend = (process.env.STORAGE_BACKEND || 'minio').toLowerCase();
+
+if (backend === 'fs') {
+  console.log('[storage] Using local filesystem backend (STORAGE_BACKEND=fs)');
+  module.exports = require('./storage-fs');
+} else {
+  module.exports = require('./minio');
+}