refactor(reader): move language normalization out of model

NormalizeLanguage lived in the model package but is a parse-time
input-cleaning helper: all of its callers are the feed-format adapters,
nothing in model uses it, and as a free function it never enforced a
model invariant. Parse-time normalization helpers belong under
internal/reader alongside date, sanitizer, and urlcleaner.

Move it to a new internal/reader/language package and rename it to
Normalize so call sites read language.Normalize(...). No behaviour
change.