refactor: drop optional POSTHOG_IP_HASH_SALT

[claude]

Requested by Elliot Hesp · Slack thread

Before / After: IP anonymization previously used an HMAC when POSTHOG_IP_HASH_SALT was set, falling back to a plain SHA-256. It now always uses a plain SHA-256, and the POSTHOG_IP_HASH_SALT env var is gone. That's one less deploy variable — the only PostHog env var is now POSTHOG_KEY.

How: Simplified anonymizeIp in app/src/lib/posthog.ts — removed the createHmac import and the salt/HMAC branch.

Note: A plain, unsalted SHA-256 of an IP is pseudonymous but technically reversible (the IPv4 space is small enough to brute-force). The privacy policy's "irreversibly hashed" wording (introduced in PR #500, docs/legal/privacy.mdx) should be softened to "hashed" to stay accurate.

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https://claude.ai/code/session_01Ud99xX7wsfWTpsu8TQxAiZ

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