Add source-IP fallback to analytics ignore check

[Gutts-n]

What changed

Added ANALYTICS_IGNORE_IPS (comma-separated) alongside the existing
ANALYTICS_IGNORE_HEADER / ANALYTICS_IGNORE_VALUES. A request from a
configured IP skips analytics logging the same way a matching header does

• either check alone is enough, they don't both need to match.

Why

Part of the NESO API-usage-metrics work (neso-nextgen-shared#152). The DXP
frontend (the public-facing NESO data portal, separate from the admin
portal and Data Explorer) 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 - callable out in #145's
original ask too ("useful, but as a secondary check... an IP-based rule
would need constant maintenance, and when it fell out of date it would
fail silently"). It's a stopgap until the header lands, not a replacement
for it.

Verified

• pytest - 488 passed.
• ruff check / mypy - clean.
• New tests: IP match skips, non-matching IP still records, header and IP
  checks are independent (either alone is sufficient).

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

[coderabbitai]

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