feat: Add company backfill migration for existing contacts (Part 1) (#12657)

## Description

Implements company backfill migration infrastructure for existing
contacts. This is **Part 1 of 2** for the company model production
rollout as described in
[CW-5726](https://linear.app/chatwoot/issue/CW-5726/company-model-setting-it-up-on-production).

Creates jobs and services to associate existing contacts with companies
based on their email domains, filtering out free email providers (gmail,
yahoo, etc.) and disposable addresses.
 

**What's included:**
- Business email detector service with ValidEmail2 (uses
`disposable_domain?` to avoid DNS lookups)
- Per-account batch job to process contacts for one account
- Orchestrator job to iterate all accounts
- Rake task: `bundle exec rake companies:backfill`

~~*NOTE*: I'm using a hard-coded approach to determine if something is a
"business" email by filtering out emails that are usually personal. I've
also added domains that are common to some of our customers' regions.
This should be simpler. I looked into `Valid_Email2` and I couldn't find
anything to dictate whether an email is a personal email or a business
one. I don't think the approach used in the frontend is valid here.~~
UPDATE: Using `email_provider_info` gem instead.


**Pending - Part 2 (separate PR):** Real-time company creation for new
contacts

## Type of change

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

## How Has This Been Tested?

```bash
# Run all new tests
bundle exec rspec spec/enterprise/services/companies/business_email_detector_service_spec.rb \\
                   spec/enterprise/jobs/migration/company_account_batch_job_spec.rb \\
                   spec/enterprise/jobs/migration/company_backfill_job_spec.rb

# Run RuboCop
bundle exec rubocop enterprise/app/services/companies/business_email_detector_service.rb \\
                     enterprise/app/jobs/migration/company_account_batch_job.rb \\
                     enterprise/app/jobs/migration/company_backfill_job.rb \\
                     lib/tasks/companies.rake
```

**Performance optimization:**
- Uses `disposable_domain?` instead of `disposable?` to avoid DNS MX
lookups (discovered via tcpdump analysis - `disposable?` was making
network calls for every email, causing 100x slowdown)

## Checklist:

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

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

db/migrate/20251021082242_add_unique_index_to_companies_domain.rb

@@ -0,0 +1,16 @@
+class AddUniqueIndexToCompaniesDomain < ActiveRecord::Migration[7.1]
+  def up
+    remove_index :companies, name: 'index_companies_on_domain_and_account_id', if_exists: true
+
+    add_index :companies, [:account_id, :domain],
+              unique: true,
+              name: 'index_companies_on_account_and_domain',
+              where: 'domain IS NOT NULL'
+  end
+
+  def down
+    remove_index :companies, name: 'index_companies_on_account_and_domain', if_exists: true
+    add_index :companies, [:domain, :account_id],
+              name: 'index_companies_on_domain_and_account_id'
+  end
+end

db/schema.rb

@@ -577,8 +577,8 @@ ActiveRecord::Schema[7.1].define(version: 2025_10_22_152158) do
     t.bigint "account_id", null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.index ["account_id", "domain"], name: "index_companies_on_account_and_domain", unique: true, where: "(domain IS NOT NULL)"
     t.index ["account_id"], name: "index_companies_on_account_id"
-    t.index ["domain", "account_id"], name: "index_companies_on_domain_and_account_id"
     t.index ["name", "account_id"], name: "index_companies_on_name_and_account_id"
   end