feat: return total posts per year in GET posts by project

[blushi]

Description

needed for: https://regennetwork.atlassian.net/browse/APP-23?atlOrigin=eyJpIjoiNGZjYTE2N2Y2YTgwNDc4Y2FjMTY5OGJkNTZkMTkxYjEiLCJwIjoiaiJ9

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Author Checklist

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