image service: photos and app icons, made on demand

GET  /healthz
POST /v1/photos {runId, slots:[{id,caption}], generationCap, style?}   202 {job, statusUrl, slots:[{id,url,statusUrl}]}
GET  /v1/photos/<job>/status · /<slot>.jpg (302 | 202 pending | 404) · /<slot>.json?wait=<s>
GET  /v1/images/<name>                              a stored picture (immutable)
POST /v1/icon {name, pitch, audience, describe, accent, style, darkStyle, count, judge, letter, motif}
                                                    app icon candidates; describe: the maker's words
POST /v1/icon/restyle {symbolId | letter, name, text, font, style, accent, darkStyle, motif}   a symbol in another style
                                                    or color, free, content-hashed
POST /v1/icon/appearances {symbolId | letter: {name, text, font, motif}, style, accent, darkStyle, tint} | {icon, tint}
                                                    the icon as phones show it (256 px, masked), free, content-hashed
GET  /v1/icon/letter?name=&accent=&style=&motif=&mirrored=   a Letter icon {iconId, symbolId, urls}, free,
                                                    deterministic
GET  /v1/icons/<id>.png                             one icon output (immutable, PNG, public)
GET  /api/admin/requests                            the request log (staff console)