[M5] Add AI 3D provider abstraction and configuration

[weiuou]

Goal

Introduce a replaceable AI image-to-3D provider layer while keeping the current procedural model generation as the safe fallback.

Scope

• Add an AI3DProvider interface for source image bytes, category, and task parameters.
• Add provider selection configuration: AI_3D_ENABLED, AI_3D_PROVIDER, timeout, retry count, Meshy API key, and local endpoint.
• Add a procedural provider that wraps the current deterministic base mesh generation.
• Normalize provider metadata into base_mesh and preview_model artifact metadata without adding new artifact kinds.
• Keep model_generating as the existing pipeline stage.

Acceptance Criteria

• With AI disabled or unconfigured, current model generation behavior is unchanged.
• Provider metadata includes model_provider, fallback_used, source_image_ids, raw_output_format, and duration fields.
• Unit tests cover provider selection, disabled mode, and procedural fallback.