feat(signup-credit): digest an identity into a keyed fingerprint

The ledger has to outlive the account that wrote it, so it cannot store a
subject id or an email. HMAC under SECRET_KEY rather than a bare hash: the
space of Google subject ids and email addresses is small enough to exhaust
offline against a plain SHA-256.

surfsense_backend/app/signup_credit/__init__.py

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+"""The signup credit: granted once per person, not once per account."""

surfsense_backend/app/signup_credit/identity/fingerprint.py

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+"""One-way, keyed digest of an identity value."""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+
+from app.config import config
+
+
+def fingerprint(value: str) -> str:
+    """Digest an identity so it stays comparable but never readable."""
+    # Keyed rather than a bare hash: the space of subject ids and email
+    # addresses is small enough to exhaust offline.
+    if not config.SECRET_KEY:
+        raise RuntimeError("SECRET_KEY must be set before identities can be claimed.")
+
+    return hmac.new(
+        config.SECRET_KEY.encode(), value.encode(), hashlib.sha256
+    ).hexdigest()