Persist anonymous users-by-location and fix IPv6 geo

[LakshmanTurlapati]

Summary

• Persist a k-anonymized last-known region on 365-day daily rollups so the stats globe still plots after the 7-day raw-event wipe, and expose it as users_by_region_365d.
• Resolve client IPs from Fly-Client-IP and look up native IPv6 against a sibling DB-IP table instead of collapsing Fly anycast/6PN onto US-CA or unknown.
• Set theme color-scheme and an opaque footer language-select background so Chrome/Firefox stop painting white-on-white in dark mode.

Test plan

• Stats globe uses 365-day regions when present and still falls back to popular_regions
• IPv6 fixture lookups and Fly-Client-IP hashing never persist plaintext IP
• Footer language picker stays readable in Chrome dark mode and light mode

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