Make ENV vars optional for multi-tenant usage

[ebrett]

Implements Option A from #7: Make ENV vars optional in global config and emphasize per-instance credential passing for multi-tenant applications.

Problem

The gem appeared to require NATIONBUILDER_CLIENT_ID and NATIONBUILDER_CLIENT_SECRET environment variables, but this was only due to the global configuration pattern shown in examples. The gem already supports passing credentials per-instance, which is the preferred pattern for multi-tenant applications.

Solution

This PR clarifies the documentation to show that:

1. ENV vars are optional - credentials can be passed directly to Client.new
2. Per-instance credentials are recommended - especially for multi-tenant apps
3. Global config is optional - only needed if you want to set defaults

Changes

Documentation Updates (README.md)

• Quick Start section: Now recommends per-instance credentials first, with global config as optional
• Configuration Options: Reorganized to show instance configuration first (recommended), followed by optional global configuration
• Multi-Tenant Usage: Moved earlier in document and enhanced with database storage example showing how to store OAuth credentials in database instead of ENV vars
• Removed duplicate: Old Multi-Tenant section that was less comprehensive

Test Updates (spec/spec_helper.rb)

• Made ENV vars optional in VCR configuration (only filters if set)
• Added comments clarifying ENV vars are NOT required for tests to run
• Tests already use per-instance credentials pattern

Benefits

✅ No ENV variables required - Credentials passed per-instance
✅ Better multi-tenant support - Store OAuth credentials in database
✅ Flexible configuration - Mix global defaults with per-instance overrides
✅ Cleaner architecture - Credentials in database, not environment
✅ All tests passing - 92.51% coverage, no ENV vars needed

Example Usage

Before (appeared to require ENV vars)

# config/initializers/nationbuilder_api.rb
NationbuilderApi.configure do |config|
  config.client_id = ENV['NATIONBUILDER_CLIENT_ID']  # Required?
  config.client_secret = ENV['NATIONBUILDER_CLIENT_SECRET']  # Required?
  config.redirect_uri = ENV['NATIONBUILDER_REDIRECT_URI']
end

After (per-instance credentials, no ENV vars needed)

# app/services/nationbuilder_service.rb
client = NationbuilderApi::Client.new(
  client_id: nation.client_id,      # From database
  client_secret: nation.client_secret,  # From database
  redirect_uri: nation.redirect_uri,
  identifier: "nation_#{nation.id}"
)

Closes #7