Render PlantUML source to SVG or PNG

The core function. Send PlantUML source text; receive the rendered diagram. format defaults to svg. PlantUML syntax errors still return an image (200) containing the error message - call POST /api/validate first (or use /api/render/batch, which validates per item) to get a machine-readable error instead. Responses carry a strong ETag and Cache-Control: public, max-age=86400, immutable; send If-None-Match to receive 304 for a repeated source. Every response carries RateLimit-* headers.