Implement a Freshdesk ticketing adapter

## 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.

Start with src/voxforge/infrastructure/providers/support/zendesk.py and the provider selection in src/voxforge/infrastructure/providers/support/factory.py, then review docs/architecture/customer-support-tools.md. Add Freshdesk coverage for ticket creation, lookup by ID and customer email, and the listed conversation metadata using mocked HTTP tests. Done means provider validation works with TICKETING_PROVIDER=freshdesk and the known-limitations, architecture, and CHANGELOG documentation is updated.

Written by the indexing model from the issue text.