Add Requesty as an OpenAI-compatible LLM provider

[Thibaultjaigu]

Adds a dedicated requesty provider mirroring the existing OpenRouter provider.

What this does

• New RequestyProvider (src/caal/llm/providers/requesty_provider.py) subclasses OpenAICompatibleProvider with base URL https://router.requesty.ai/v1. Auth is Authorization: Bearer <key>, plus the same HTTP-Referer / X-Title attribution headers the OpenRouter provider sets.
• Registered in both factories in src/caal/llm/providers/__init__.py: create_provider("requesty", ...) and create_provider_from_settings(...) (api_key from requesty_api_key setting or REQUESTY_API_KEY env; model from requesty_model, default openai/gpt-4o-mini). Supported-provider strings in docstrings and both ValueError messages updated.
• Model naming is provider/model (same convention as OpenRouter), e.g. openai/gpt-4o-mini, anthropic/claude-sonnet-4-5.

Config sites mirrored (every place OpenRouter appears for config)

• src/caal/llm/providers/requesty_provider.py (new)
• src/caal/llm/providers/__init__.py (imports, __all__, both factories, docstrings, error messages)
• src/caal/settings.py (requesty_api_key / requesty_model defaults)
• voice_agent.py (runtime config block + startup log line)
• src/caal/chat/api.py (runtime config block + reload model resolution)
• .env.example (LLM_PROVIDER=requesty choice + Requesty block)
• README.md (provider mentions + acknowledgements list)

Live test

Tested live against https://router.requesty.ai/v1: a real chat completion with openai/gpt-4o-mini (the same request shape RequestyProvider produces — bearer auth + the attribution headers) returned HTTP 200 with a valid completion.

Notes

This intentionally does not touch the frontend setup wizard / settings UI, the webhooks.py /setup/test-* endpoint, or the i18n message JSONs (frontend/messages/*.json, mobile l10n). Those can follow in a separate change if wanted.

Docs: https://requesty.ai , https://docs.requesty.ai , https://app.requesty.ai/api-keys , https://app.requesty.ai/router/list

I work at Requesty. This mirrors the existing OpenRouter provider as closely as possible. Happy to adjust wording/placement or close it if it's not a fit.