ImageProcessor.Process used io.ReadAll without a size limit, allowing
arbitrarily large inputs to exhaust memory. Add a configurable
maxInputBytes limit (default 50 MiB, matching the fetcher limit) and
reject inputs that exceed it with ErrInputDataTooLarge.
Also bound the cached source content read in the service layer to
prevent unexpectedly large cached files from consuming unbounded memory.
Extracted loadCachedSource helper to reduce nesting complexity.
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