Add Rails backend: accounts via magic link, server documents, sharing

The app becomes a Rails 8 application; the Vue frontend moves to
app/frontend and is served through vite_rails.

- Without an account documents stay in the browser (IndexedDB). Sign-in
  is by magic link or the 6-digit code from the same mail; afterwards
  local documents can be uploaded into the account.
- Server documents (/d/<id>) with versioned saves and a single-editor
  lock per browser tab; others see them read-only and follow by polling.
- Sharing via link (works without an account) and via e-mail invitation,
  roles viewer/editor; the owner manages links and people.
- Store gains a read-only mode that blocks all content changes.
- Solid Queue, Cache and Cable share the one PostgreSQL database.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

db/migrate/20260928120200_create_solid_cache_tables.rb

@@ -0,0 +1,16 @@
+# Solid Cache lives in the main database (same as the app data),
+# converted from the generated db/cache_schema.rb.
+class CreateSolidCacheTables < ActiveRecord::Migration[8.1]
+  def change
+    create_table "solid_cache_entries" do |t|
+      t.binary "key", limit: 1024, null: false
+      t.binary "value", limit: 536870912, null: false
+      t.datetime "created_at", null: false
+      t.integer "key_hash", limit: 8, null: false
+      t.integer "byte_size", limit: 4, null: false
+      t.index ["byte_size"], name: "index_solid_cache_entries_on_byte_size"
+      t.index ["key_hash", "byte_size"], name: "index_solid_cache_entries_on_key_hash_and_byte_size"
+      t.index ["key_hash"], name: "index_solid_cache_entries_on_key_hash", unique: true
+    end
+  end
+end