* feat(artwork): make the artwork image size cap configurable
Replace the hardcoded 20MB cap on resolved image reads with a new
MaxImageSize config option. Load floors it at MaxImageUploadSize so an
accepted upload can never be too large for the resolver to read back.
* fix(conf): reject zero-valued byte-size options at startup
ParseBytes accepts "0", but parseSize silently substitutes the default
for it, so the accepted config would differ from the effective limit.
* fix(conf): reject byte-size options that overflow int64
A raw value above math.MaxInt64 parses as a valid uint64 but wraps to a
negative int64 in parseSize, giving readCapped a non-positive LimitReader
bound so every artwork read comes back empty.
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