Implement a Freshdesk ticketing adapter #23

Summary

Zendesk create/lookup exists with unit tests. Freshdesk is still a blocked stub (Ticketing provider 'freshdesk' was removed).

Context

• Roadmap v1.0: Freshdesk adapter
• src/voxforge/infrastructure/providers/support/factory.py
• src/voxforge/infrastructure/providers/support/zendesk.py (pattern to follow)
• docs/architecture/customer-support-tools.md
• Env already has FRESHDESK_DOMAIN / FRESHDESK_API_KEY in config

Scope

• Backend adapter: create ticket, lookup by id, lookup by customer email
• Carry conversation summary, replay URL, session id (same as Zendesk)
• Duplicate suppression if Freshdesk supports tags/unique keys
• Production validation when TICKETING_PROVIDER=freshdesk
• Unit tests (httpretty/respx/httpx mock, no live account required in PR CI)
• Docs: known-limitations, customer-support-tools, CHANGELOG

Out of scope

Live sandbox proof can be a follow-up, same as Zendesk.