feat(email): alert and weekly-report emails read as designed messages (#5335)

#5335 "Emails Steward sends read as designed messages: an HTML alert email
and weekly report, with a plain-text twin".

- core/mail.py renders each email as multipart/alternative from
  templates/email/: a table-and-inline-style HTML part and a plain-text
  twin carrying the same facts. Colours are the design system's light
  theme, resolved from theme.css; nothing is fetched when a mail opens.
- Alert email: state badge, a headline sentence ("Disk used on web01 is
  94%."), rule, resource, reading, the condition in words, and the time in
  the zone Notifications was saved in with UTC beside it; "View in
  Steward" when Steward's address is set. Subject "[Steward] Firing: <rule>
  — <resource> 94%". Alerts now carry their operator.
- Weekly report: monitors down, active alerts, the week's alert events,
  uptime and busiest Traefik routers as small tables with links. Sent from
  the configured From address; the "Generated … UTC" line and the stale
  repo footer are gone.
- condition_text() states a condition from a rule's parts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/test_notifications.py

@@ -9,7 +9,7 @@ import pytest
 from steward.core import notifications as n
 
 ALERT = {"rule_name": 'Disk "root" on\nweb01 \\ backslash', "state": "FIRING", "metric": "disk_used_pct",
-         "value": 93.5, "threshold": 90.0, "resource": "web01", "source_module": "host_agent",
+         "value": 93.5, "operator": ">=", "threshold": 90.0, "resource": "web01", "source_module": "host_agent",
          "note": None, "timestamp": "2026-10-06T12:00:00+00:00"}
 
 
@@ -116,13 +116,7 @@ async def test_email_from_falls_back_and_a_line_break_in_the_name_still_sends(mo
         assert (await n._send_email(cfg, ALERT))["sent"]
     assert [f for f, _ in sent] == ["alerts@example.com", "u@example.com", "steward@localhost"]
     # The rule name has a line break; the subject folds it to a space rather than failing.
-    assert sent[0][1] == '[Steward] FIRING — Disk "root" on web01 \\ backslash'
-
-
-def test_email_body_names_no_data_and_the_note():
-    body = n.email_body({**ALERT, "value": None, "note": "no sample for 18m"})
-    assert "Metric: disk_used_pct = no data\n" in body and "Note: no sample for 18m\n" in body
-    assert "Note:" not in n.email_body(ALERT)
+    assert sent[0][1] == '[Steward] Firing: Disk "root" on web01 \\ backslash — web01 94%'
 
 
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