Revert partial-month cutoff to Utc::now()

PR #29 changed the cutoff from Utc::now() to MAX(created_at) under the
theory that data fetched mid-month would otherwise show partial counts.
But in the common case where data is fully fetched through last month
and no PRs have been created yet in the current calendar month,
MAX(created_at) returns last month — so the last complete month gets
silently dropped.

Concrete regression: on 2026-05-24 the vector PR data goes through
2026-04 (a complete month), but MAX(created_at)='2026-04' was excluding
all of April from the contributor heatmap and unique-contributors chart.
Validated against the DB: April actually has 25 unique non-bot
non-draft contributors with the standard exclusions (matches the new
chart exactly).

Reverting to Utc::now() handles both cases correctly: mid-month fetches
still get the current month excluded (since Utc::now() == that month),
and a stale fetch of a complete month is kept.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: pront