Fail-closed is_admin resolver (Feature 03, task 5)

admin_allowlist.is_admin: True only for an allowlisted HA user id;
False for non-allowlisted, unknown, empty-string, untrimmed variants,
None, non-strings (bools, lists), and an empty allowlist (nobody is
admin rather than everybody). Never raises for malformed input —
malformed means not admin; DB errors propagate for the Feature 09
gate to handle. The last-admin removal refusal (set_admin_ids) is
re-asserted as keeping the verdict sound. Tests: full verdict matrix,
empty-allowlist fail-closed, refusal persistence. Suite: 912 passed.

custom_components/nestquest/admin_allowlist.py

@@ -91,6 +91,23 @@ async def set_admin_ids(
                 await dao.add(user_id, _now_stamp())
 
 
+async def is_admin(
+    database: NestQuestDatabase, user_id: object
+) -> bool:
+    """Resolve an HA user ID against the allowlist — FAILS CLOSED.
+
+    True only for an allowlisted user id.  Everything else is False:
+    a non-allowlisted id, an unknown id, a None or non-string value
+    (a service call with no user context), and an empty allowlist
+    (nobody is admin rather than everybody).  The Feature 09 gate
+    wraps every admin-only service with this verdict; it never raises
+    for malformed input — malformed means not admin.
+    """
+    if not isinstance(user_id, str) or not user_id:
+        return False
+    return await AdminUsersDao(database).exists(user_id)
+
+
 async def seed_setup_admin(
     database: NestQuestDatabase,
     *,

tests/test_admin_allowlist.py

@@ -415,3 +415,57 @@ async def test_reopened_setup_failure_closes_database(
     # The reopen path opened AND closed a connection; the failure is
     # the last thing observed.
     assert close_calls[-1] is True
+
+
+# ---------------------------------------------------------------------------
+# is_admin: the fail-closed resolver (Feature 09's verdict function)
+# ---------------------------------------------------------------------------
+
+
+def test_is_admin_verdict_matrix(tmp_path) -> None:
+    async def _body(database):
+        from custom_components.nestquest.admin_allowlist import is_admin
+
+        await set_admin_ids(database, ["user-owner"])
+        assert await is_admin(database, "user-owner") is True
+        assert await is_admin(database, "user-other") is False
+        # Unknown, malformed, and absent identities all fail closed.
+        assert await is_admin(database, "") is False
+        assert await is_admin(database, "  user-owner  ") is False, (
+            "ids are stored trimmed; an untrimmed lookup is a different "
+            "id and must not match"
+        )
+        assert await is_admin(database, None) is False
+        assert await is_admin(database, 42) is False
+        assert await is_admin(database, ["user-owner"]) is False
+        assert await is_admin(database, True) is False
+        return None
+
+    _with_db(tmp_path, "resolver-matrix.db")(_body)
+
+
+def test_is_admin_empty_allowlist_fails_closed(tmp_path) -> None:
+    """An empty allowlist returns False for EVERYONE, not True."""
+    async def _body(database):
+        from custom_components.nestquest.admin_allowlist import is_admin
+
+        for candidate in ("user-owner", "", None, 1):
+            assert await is_admin(database, candidate) is False
+        return None
+
+    _with_db(tmp_path, "resolver-empty.db")(_body)
+
+
+def test_is_admin_false_after_last_admin_removal_attempt(tmp_path) -> None:
+    """The refusal keeps the verdict sound: a refused removal leaves
+    the admin in place, so is_admin still resolves True."""
+    async def _body(database):
+        from custom_components.nestquest.admin_allowlist import is_admin
+
+        await set_admin_ids(database, ["user-owner"])
+        with pytest.raises(ValueError, match="at least one admin"):
+            await set_admin_ids(database, [])
+        assert await is_admin(database, "user-owner") is True
+        return None
+
+    _with_db(tmp_path, "resolver-refusal.db")(_body)