feat(cli): support any OpenAI-compatible vision endpoint for image captioning

[yuemeng200]

Summary

The hyperframes capture image captioning feature currently supports only two hardcoded providers: Google Gemini and OpenRouter. This blocks users outside of Google's supported regions (e.g. mainland China) from getting AI-driven asset descriptions, since neither Gemini nor OpenRouter is easily accessible there.

This PR adds three new environment variables that let any OpenAI-compatible vision endpoint be used:

Variable	Description
HYPERFRAMES_VISION_API_KEY	Bearer token for the custom endpoint
HYPERFRAMES_VISION_BASE_URL	Base URL (e.g. https://ark.cn-beijing.volces.com/api/v3)
HYPERFRAMES_VISION_MODEL	Model ID (required; warns and skips if unset)

Provider priority (first match wins):

1. HYPERFRAMES_VISION_* — custom OpenAI-compatible endpoint ← new
2. OPENROUTER_API_KEY — OpenRouter (unchanged)
3. GEMINI_API_KEY / GOOGLE_API_KEY — Google Gemini (unchanged)

Example (Volcengine ARK / Doubao vision):

HYPERFRAMES_VISION_API_KEY=ark-xxx
HYPERFRAMES_VISION_BASE_URL=https://ark.cn-beijing.volces.com/api/v3
HYPERFRAMES_VISION_MODEL=doubao-seed-2-0-mini-260428

Works equally well with Azure OpenAI, local Ollama, self-hosted vLLM, or any other endpoint that speaks the OpenAI chat completions wire format with image_url content parts.

Implementation notes

• The OpenRouter fetch path is refactored to share the new openAiCompatCaptionOne helper — no behaviour change for existing OpenRouter users.
• If HYPERFRAMES_VISION_API_KEY and HYPERFRAMES_VISION_BASE_URL are set but HYPERFRAMES_VISION_MODEL is missing, a warning is pushed and captioning is skipped gracefully (same degradation pattern as a missing key).

Tests

4 new unit tests added to contentExtractor.test.ts:

• Custom endpoint happy path (verifies URL, auth header, model, base64 image payload)
• Custom endpoint takes priority over OPENROUTER_API_KEY when both are set
• Missing HYPERFRAMES_VISION_MODEL → warning emitted, fetch not called
• No-key path now also stubs HYPERFRAMES_VISION_API_KEY for completeness

All 6 tests pass (bunx vitest run). Build clean (bun run build). Lint + format clean (oxlint + oxfmt).