email: render_digest_email — multipart digest template

Adds render_digest_email(kind, date_str, content_html, unsubscribe_url,
settings_url) -> tuple[str, str, str] to email_service.py, following the
same contract as render_otp_email. Includes _DIGEST_HTML_TEMPLATE with
light/dark palette from branding and _strip_html_to_text for the plain-text
fallback. Unit tests in tests/test_email_render.py cover daily, weekly, and
invalid-kind cases.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_email_render.py

@@ -0,0 +1,44 @@
+"""Unit tests for render_digest_email."""
+from __future__ import annotations
+
+from app.services.email_service import render_digest_email
+
+
+def test_daily_subject_and_bodies():
+    subj, text, html = render_digest_email(
+        kind="daily",
+        date_str="2026-05-25",
+        content_html="<p>Markets did stuff today.</p>",
+        unsubscribe_url="https://read.markets/email/unsubscribe?token=abc",
+        settings_url="https://read.markets/settings",
+    )
+    assert "Daily" in subj
+    assert "2026-05-25" in subj
+    assert "Markets did stuff today" in html
+    assert "abc" in html  # unsubscribe link landed
+    assert "/settings" in html
+    # Plain-text fallback strips HTML.
+    assert "<p>" not in text
+    assert "Markets did stuff today" in text
+
+
+def test_weekly_subject_says_recap():
+    subj, _, _ = render_digest_email(
+        kind="weekly",
+        date_str="2026-05-25",
+        content_html="<p>x</p>",
+        unsubscribe_url="https://x/u",
+        settings_url="https://x/s",
+    )
+    assert "Weekly" in subj
+    assert "recap" in subj.lower()
+
+
+def test_invalid_kind_raises():
+    import pytest
+    with pytest.raises(ValueError):
+        render_digest_email(
+            kind="bogus", date_str="2026-05-25",
+            content_html="<p>x</p>",
+            unsubscribe_url="u", settings_url="s",
+        )