Store the remoteStorage user agent as text, not varchar(255)

Real-world user agents (in-app browsers, Android WebView variants) commonly
exceed 255 characters. On PostgreSQL that would raise and 500 the consent
POST and token exchange; SQLite dev/test does not enforce the limit, so it
went unnoticed. Use text columns for user_agent on both tables.

Part of #254

db/migrate/20261102130000_add_user_agent_to_rs_authorizations.rb

@@ -1,6 +1,6 @@
 class AddUserAgentToRsAuthorizations < ActiveRecord::Migration[8.1]
   def change
-    add_column :remote_storage_authorizations, :user_agent, :string
-    add_column :rs_authorization_codes, :user_agent, :string
+    add_column :remote_storage_authorizations, :user_agent, :text
+    add_column :rs_authorization_codes, :user_agent, :text
   end
 end

db/schema.rb

@@ -110,7 +110,7 @@ ActiveRecord::Schema[8.1].define(version: 2026_11_02_130000) do
     t.string "redirect_uri"
     t.string "token"
     t.datetime "updated_at", null: false
-    t.string "user_agent"
+    t.text "user_agent"
     t.integer "user_id", null: false
     t.integer "web_app_id"
     t.index ["permissions"], name: "index_remote_storage_authorizations_on_permissions"
@@ -129,7 +129,7 @@ ActiveRecord::Schema[8.1].define(version: 2026_11_02_130000) do
     t.text "permissions", default: "[]"
     t.string "redirect_uri", null: false
     t.datetime "updated_at", null: false
-    t.string "user_agent"
+    t.text "user_agent"
     t.integer "user_id", null: false
     t.index ["code_digest"], name: "index_rs_authorization_codes_on_code_digest", unique: true
     t.index ["user_id"], name: "index_rs_authorization_codes_on_user_id"