Use dt.timezone.utc so the declared >=3.10 support is real

`dt.UTC` is a 3.11+ alias, but crdt-sync declares requires-python = ">=3.10"
and the CI matrix tests 3.10 — so `crdt-sync tests` has been failing on the
3.10 leg since before this branch (commit 493db693 failed the same way).
It collected as `AttributeError: module 'datetime' has no attribute 'UTC'`.

This is not only a test problem: `_firebase_auth._utcnow()` is library code,
so on 3.10 every token-expiry check would have raised at runtime. The three
consumers (diet-guard, screen-locker, wake-alarm) all declare >=3.10 too, so
honouring the floor is the right direction rather than raising it.

`dt.UTC is dt.timezone.utc` evaluates to True — the same singleton under two
spellings — so this is a pure rename with no behaviour change.

Verified on a real pyenv 3.10.16 venv, not just the 3.14 dev interpreter:
239 passed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GZH89WEqAjuJT8WZdqc7AP

crdt-sync/crdt_sync/_firebase_auth.py

@@ -161,7 +161,7 @@ class FileCredentialStore:
 
 
 def _utcnow() -> dt.datetime:
-    return dt.datetime.now(dt.UTC)
+    return dt.datetime.now(dt.timezone.utc)
 
 
 class FirebaseTokenProvider:

crdt-sync/crdt_sync/tests/test_firebase.py

@@ -56,7 +56,7 @@ def _auth(*, signed_in: bool = True) -> FirebaseTokenProvider:
         FirebaseCredentials(
             id_token=_FAKE_ID,
             refresh_token=_FAKE_REFRESH,
-            expires_at=dt.datetime(2099, 1, 1, tzinfo=dt.UTC),
+            expires_at=dt.datetime(2099, 1, 1, tzinfo=dt.timezone.utc),
         )
         if signed_in
         else None

crdt-sync/crdt_sync/tests/test_firebase_auth.py

@@ -27,7 +27,7 @@ from crdt_sync import _firebase_auth as fa
 if TYPE_CHECKING:
     from pathlib import Path
 
-_NOW = dt.datetime(2026, 8, 3, 12, tzinfo=dt.UTC)
+_NOW = dt.datetime(2026, 8, 3, 12, tzinfo=dt.timezone.utc)
 
 # Fixture values referenced through names ruff's hardcoded-credential checks
 # (S105-S107) do not key off. Comparing an attribute called `id_token` to a