notion-react: add coverage for media caption + file_upload

Part of v0.1 block-coverage epic (see context/notion-block-coverage/).

Problem

Image / Video / Audio / File / Pdf components (components/blocks.tsx) only project { url } as { type: 'external', external: { url } }. Missing:

• caption (rich_text) — lossy for any page imported from Notion.
• file_upload variant — the 2026-era successor to internal file uploads; should be pluggable without schema churn.

Deliverables

• components/props.ts: extend MediaProps with caption?: ReactNode and fileUploadId?: string.
• components/blocks.tsx + renderer/host-config.ts: emit caption: rich_text[] and { type: 'file_upload', file_upload: { id } } envelope when fileUploadId is set; keep url->external path.
• web/blocks.tsx: already renders image caption — extend to video/audio/file/pdf.
• E2E: extend Image test to cover caption; add Video, Audio, File, Pdf round-trips.

Acceptance

• Round-trip preserves caption.
• file_upload path works against a live upload id (or is at least unit-tested for payload shape).

Start with components/props.ts, components/blocks.tsx, and renderer/host-config.ts to trace MediaProps and the existing URL-to-external payload. Then inspect web/blocks.tsx and the existing Image E2E test before extending coverage to video, audio, file, and PDF. Done means captions survive round-trips and the file_upload payload shape is tested or works with a live upload id.