Sync v1 REST client with API spec (/v1/assets)

[ellipsis-dev]

Syncs the hand-maintained /v1 REST client with the API spec after ellipsis-dev/ellipsis#5634 (Agent asset storage: backend upload API + gated viewing route), which added the /v1/assets surface.

Only src/lib/api.ts and src/lib/types.ts changed.

Endpoints added

• POST /v1/assets → createAsset(req: CreateAssetRequest): Promise<CreateAssetResponse> — upload a base64 PNG (≤10 MiB), returns the gated dashboard URL.
• GET /v1/assets → listAssets(query?: ListAssetsQuery): Promise<AssetView[]> — newest-first metadata for the token's customer; optional agent_session_id filter and limit. Unwraps ListAssetsResponse.assets (matches the listAgentSessions pattern).
• GET /v1/assets/{asset_id} → getAsset(assetId): Promise<GetAssetResponse> — metadata + gated url + short-lived presigned download_url.

Types added

• AssetView — id, filename, content_type, size_bytes, created_at, optional agent_session_id.
• CreateAssetRequest — filename, content_type, data_b64.
• CreateAssetResponse — asset, url.
• ListAssetsQuery — agent_session_id?, limit?.
• ListAssetsResponse — assets.
• GetAssetResponse — asset, url, download_url.

Out of scope

• GET /frontend/assets/{asset_id} is a /frontend viewer route, not part of the public /v1 surface — intentionally not mirrored.
• No run-stream / WebSocket frame schema changes in this PR, so src/lib/ws.ts is untouched.

Verification

bun run typecheck clean; bun run test → 145 passed.