feat(prefab): optional PhotoBuilder (e.g. Gemini) provider and API key

[manuelkiessling]

Summary

Allows prefab entries in prefabs.yaml to optionally set dedicated PhotoBuilder provider and API key (e.g. Google Gemini for image generation), so that projects created from that prefab use them instead of the content-editing LLM settings.

Changes

• prefabs.yaml.example: Document optional photo_builder_llm_model_provider and photo_builder_llm_api_key; add commented example for Gemini.
• PrefabDto: New optional photoBuilderLlmModelProvider and photoBuilderLlmApiKey (both null when omitted).
• PrefabLoader: Parse and validate: both keys must be set or both omitted; provider must be openai/google and support PhotoBuilder; skip invalid entries with warning.
• ProjectMgmtFacade: Pass prefab PhotoBuilder provider and key into ProjectService::create() when set (no longer always null).
• Tests: PrefabLoaderTest — load with dedicated PhotoBuilder keys; skip when only one key set; skip when invalid provider.
• Docs: docs/llm-usage-book.md — one sentence that prefabs can optionally set dedicated PhotoBuilder in prefabs.yaml.

Behaviour

• When both optional keys are omitted: unchanged (PhotoBuilder uses content-editing settings).
• When both are set (e.g. photo_builder_llm_model_provider: "google", photo_builder_llm_api_key: "..."): the created project gets dedicated PhotoBuilder (Gemini image + prompt models) with that key. Existing keys_visible applies to this key as well.

Quality and tests (PHP + frontend) run successfully.