Add source-IP fallback to analytics ignore check

[Gutts-n]

What changed

Added CKANEXT_ANALYTICS_IGNORE_IPS (comma-separated) alongside the
existing CKANEXT_ANALYTICS_IGNORE_HEADER / CKANEXT_ANALYTICS_IGNORE_VALUES.
A request from a configured IP skips analytics logging the same way a
matching header does - either check alone is enough. Also refactored
RequestEvent.request_ip to share its resolution logic with the new
is_ignored check (_request_ip static helper) instead of duplicating it.

Why

Part of the NESO API-usage-metrics work (neso-nextgen-shared#152). The DXP
frontend still doesn't send an identifying header - that part of the
original ask (neso-nextgen-shared#145) is still open. In the meantime, the
DXP team reported their web server's static egress IPs for production:
https://github.com/datopian/client-national-grid-shared/issues/496#issuecomment-3158622385
(54.247.74.82, 63.32.18.228).

IP is explicitly a weaker, secondary signal - #145's original ask called
this out too. Stopgap until the header lands, not a replacement for it.
Same change applied to datopian/datastore#21.

Verified

• pytest ckanext/analytics/tests/test_event.py - 51 passed (minimal
  flask+pytest venv, --ckan-ini addopts overridden).
• New tests: IP match skips, non-matching IP still records, header and IP
  checks are independent.

Not verified: end-to-end against a live pipeline, and I have no way to
confirm those two IPs are still current - production traffic isn't
ingested into the monitoring stack I have access to. Whoever sets
CKANEXT_ANALYTICS_IGNORE_IPS should reconfirm with the DXP team first.

[coderabbitai]

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