Support PlotToon-friendly multi-image upload limits

[realproject7]

Context

PlotToon episodes upload one final image per cut, then publish markdown referencing those IPFS image URLs. A normal webtoon episode needs 10-20 image uploads in one publishing session.

Related PlotToon planning: realproject7/plottoon#44 and realproject7/plottoon#42.

Problem

The existing single-image endpoint (POST /api/upload-plot-image) has a 5/minute rate limit per wallet. A 15-cut episode would require waiting across several minutes.

Solution: Batch upload endpoint

Add POST /api/upload-plot-images (plural) instead of raising the global single-image rate limit.

Behavior

• Accepts multiple files in one FormData request (e.g., file_0, file_1, ... or files[])
• One wallet signature covers the entire batch
• Each file validated individually: WebP/JPEG only, max 1MB, magic byte check
• Returns ordered array of results: [{ index, cid, url, mimeType, sizeBytes }]
• Failed individual files return error in their slot without failing the whole batch

Rate limiting

• Max 20 files per batch request
• Max 3 batch requests per minute per wallet
• Existing single-image endpoint (/api/upload-plot-image) unchanged — keeps 5/min limit

What to skip for MVP

• No cutId metadata field — just return results in upload order
• No purpose field — keep it simple

Acceptance criteria

• PlotToon can upload a 10-20 cut episode in one request
• Each file individually validated (type, size, magic bytes)
• Ordered results array matches upload order
• Existing single-image illustration endpoint unchanged
• Abuse protection: batch size + rate limiting enforced
• No secrets or credentials exposed