Add Magnific (magnific.com) as a provider: video, image, upscale, and audio

Problem / motivation

Covering video, image, upscaling and audio today means setting up several providers, each with its own API key. Two capabilities have no cloud provider at all:

• image_upscale: no cloud upscaler
• sound_effects: no cloud SFX generation

The closest multi-model provider, higgsfield_video, only does video.

Proposed solution

Add Magnific (formerly the Freepik API) as a provider. One MAGNIFIC_API_KEY would cover:

• Video: Seedance 2, Kling v3, Veo 3.1, WAN, LTX, Hailuo, Runway, PixVerse
• Image: Mystic, Flux 2, Seedream 5
• Upscale: Magnific Creative and Precision upscalers
• Audio: ElevenLabs music, sound effects, and SAM Audio isolation

Every Magnific endpoint uses the same async submit/poll/download flow, so one shared client (tools/_magnific.py) can back six tools: magnific_video, magnific_image, magnific_upscale, magnific_music, magnific_sound_effects, magnific_audio_isolation.

The registry finds the tools by capability, so the selectors don't need to change.

Implementation: #628

Alternatives considered

• Magnific's MCP server (https://mcp.magnific.com, OAuth, no API key): fine for one-off work in chat, but pipelines need tools they can call directly and track costs for. The PR adds a section to docs/PROVIDERS.md on when to use which.
• Keep one provider per capability: means more keys to manage, and still leaves upscaling and SFX with no cloud option.

Additional context

• Magnific bills in credits and doesn't publish a price per endpoint, so estimate_cost gives approximate USD for budget checks only. The "Unlimited" plan covers the web app, not the API.
• Follows the same gateway pattern as #569.
• PR: https://github.com/calesthio/OpenMontage/pull/628