A send that fails for a passing reason is retried; one that finally fails is marked on the draft (#242)

Every sender turned every failure into `false`, so a one-minute Gmail
rate limit or a short outage lost the send like a refused recipient.

The three senders now classify through MailDeliveryInterface::deliver():
deferred (rate limits, 5xx, a connection that never opened, SMTP 4xx),
refused (other 4xx, SMTP 5xx, too large) or outcome unknown (a lost
answer, 502/504). send() stays a bool for invitations, alerts and read
receipts.

SendMessageHandler retries a deferral for at most two minutes, keeping
the provider's Retry-After. A lost answer is never resent blind: Gmail
and Graph look for the Message-ID in Sent first (now committed before
the first attempt), SMTP reports it as possibly sent. A final failure,
from the handler or from SendGaveUpSubscriber when the ladder runs out,
marks the draft, words the toast by its reason, and shows a red warning
triangle at the start of the draft's row and on Drafts in the sidebar.

Migration: four nullable columns and a partial index on message.

Closes #125

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

templates/_partials/_sidebar.html.twig

@@ -552,6 +552,8 @@ controller instead — the scroll position and which trees are open.
                 <i class="fa-solid fa-file-lines w-4" aria-hidden="true"></i>
                 <span class="rail-hide">{{ 'sidebar.nav.drafts'|trans }}</span>
                 {{ macros.role_dot(enum('App\\Domain\\Enum\\Mail\\LabelRole').Drafts) }}
+                {% import '_partials/_unsent_mark.html.twig' as unsent %}
+                {{ unsent.sidebar(sidebarCounts.unsentDrafts()) }}
             </span>
             {{ macros.total_badge(sidebarCounts.forRoleBadge(enum('App\\Domain\\Enum\\Mail\\LabelRole').Drafts), 'role:drafts') }}
         </a>

templates/_partials/_thread_row.html.twig

@@ -437,6 +437,16 @@ be a worse regression than the one this feature fixes.
                    different places depending on how wide the list happens to
                    be. #}
                 <span class="min-w-0 flex items-center gap-1.5">
+                    {# A send of this draft that finally failed (#125): first in
+                       the marker slot, at the very start of the row, where a
+                       glance down the Drafts list finds it before anything
+                       else. See _unsent_mark.html.twig; `draftMessage` is only
+                       set on a conversation row. #}
+                    {% if draftMessage|default(null) and draftMessage.sendFailure %}
+                        {% import '_partials/_unsent_mark.html.twig' as unsent %}
+                        {{ unsent.row(draftMessage.sendFailure) }}
+                    {% endif %}
+
                     {# The "New" marker: a filled accent pill carrying a WORD.
                        Deliberately nothing like the unread dot further down
                        this file, which is a bare 8px green circle — the two

templates/_partials/_unsent_mark.html.twig

@@ -0,0 +1,32 @@
+{#
+The warning on a draft whose send finally failed (#125) — on its row in the
+Drafts list, and on the Drafts entry in the sidebar while any draft has one.
+
+One icon in both places, defined once, so the sidebar's mark is plainly the
+same thing as the row's and the reader knows what to look for when they open
+Drafts. Red, because it is the one state here that means "this did not happen
+and you need to know": the toast that said so is gone by the time a scheduled
+send's failure is noticed.
+
+The sidebar's is rendered at every count and hidden at zero, the idiom every
+patched marker follows (see _new_dot.html.twig): ui--sidebar finds it by
+`data-count-key` after a counts refresh, and an element never rendered cannot
+be found.
+#}
+{% macro row(failure) %}
+    {% set reason = ('thread_row.send_failed.' ~ failure.value)|trans %}
+    <i class="fa-solid fa-triangle-exclamation shrink-0 text-red-600 dark:text-red-400"
+       role="img"
+       data-unsent-draft
+       title="{{ reason }}"
+       aria-label="{{ reason }}"></i>
+{% endmacro %}
+
+{% macro sidebar(count) %}
+    <i class="fa-solid fa-triangle-exclamation text-red-600 dark:text-red-400 text-xs{{ count > 0 ? '' : ' hidden' }}"
+       role="img"
+       data-unsent-mark
+       data-count-key="unsent:role:drafts"
+       title="{{ 'sidebar.badge.unsent'|trans }}"
+       aria-label="{{ 'sidebar.badge.unsent'|trans }}"></i>
+{% endmacro %}

templates/mail/_send_outcome.stream.html.twig

@@ -16,14 +16,20 @@ ten. So the confirmation preceded the attempt by two seconds, every time, for
 everybody, and a send that then failed was never mentioned again.
 
 Variables: message, sent (bool)
+
+A failure is worded by its reason (#125), read off the row the handler has just
+marked: "may have been sent" and "the server refused it" ask different things of
+the reader. Failed — the reason with nothing more to say — keeps the old words.
 #}
+{% set failure = sent ? null : message.sendFailure %}
+{% set failedKey = failure and failure.value != 'failed' ? 'toast.send_failed_' ~ failure.value : 'toast.send_failed' %}
 <turbo-stream action="append" target="toast-region">
     <template>
         {% include '_partials/_toast.html.twig' with {
             type:    sent ? 'success' : 'error',
             message: sent
                 ? 'toast.sent'|trans
-                : 'toast.send_failed'|trans({ '%subject%': message.subject|default('')|slice(0, 60) }),
+                : failedKey|trans({ '%subject%': message.subject|default('')|slice(0, 60) }),
             extra:   sent ? {} : { hide_after: 12000 },
         } only %}
     </template>