feat: add per-inbox toggle to disable incoming calls (#14645)

## Description

Adds a per-inbox "Allow incoming calls" toggle for voice-enabled
WhatsApp and Twilio inboxes. When turned off, the setting is persisted
on the channel; actually rejecting inbound calls is handled in a
follow-up PR.

## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## Screenshot
<img width="804" height="384" alt="Screenshot 2026-06-04 at 11 44 07 AM"
src="https://github.com/user-attachments/assets/df8bb026-0387-4031-bcba-6d9a56872eb7"
/>


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

db/migrate/20260604000000_add_provider_config_to_channel_twilio_sms.rb

@@ -0,0 +1,5 @@
+class AddProviderConfigToChannelTwilioSms < ActiveRecord::Migration[7.1]
+  def change
+    add_column :channel_twilio_sms, :provider_config, :jsonb, default: {}
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2026_05_25_093000) do
+ActiveRecord::Schema[7.1].define(version: 2026_06_04_000000) do
   # These extensions should be enabled to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"
@@ -557,6 +557,7 @@ ActiveRecord::Schema[7.1].define(version: 2026_05_25_093000) do
     t.boolean "voice_enabled", default: false, null: false
     t.string "twiml_app_sid"
     t.string "api_key_secret"
+    t.jsonb "provider_config", default: {}
     t.index ["account_sid", "phone_number"], name: "index_channel_twilio_sms_on_account_sid_and_phone_number", unique: true
     t.index ["messaging_service_sid"], name: "index_channel_twilio_sms_on_messaging_service_sid", unique: true
     t.index ["phone_number"], name: "index_channel_twilio_sms_on_phone_number", unique: true