✨(backend) add email notifications for screen recordings

Implement backend method to send email notifications when screen recordings
are ready for download. Enables users to be alerted when their recordings are
available. Frontend implementation to follow in upcoming commits.

This service is triggered by the storage hook from Minio.

Add minimal unit test coverage for notification service, addressing previous
lack of tests in this area. The notification service was responsible for
calling the unstable summary service feature, which was developped way too
quickly.

The email template has been reviewed by a LLM, to make it user-friendly and
crystal clear.

src/mail/mjml/screen_recording.mjml

@@ -0,0 +1,67 @@
+<mjml>
+  <mj-include path="./partial/header.mjml" />
+
+  <mj-body mj-class="bg--blue-100">
+    <mj-wrapper css-class="wrapper" padding="5px 25px 0px 25px">
+      <mj-section css-class="wrapper-logo">
+        <mj-column>
+          <mj-image
+            align="center"
+            src="{{logo_img}}"
+            width="320px"
+            alt="{%trans 'Logo email' %}"
+          />
+        </mj-column>
+      </mj-section>
+      <mj-section mj-class="bg--white-100" padding="0px 20px 60px 20px">
+        <mj-column>
+          <mj-text align="center">
+            <h1>{% trans "Your recording is ready!"%}</h1>
+          </mj-text>
+          <!-- Main Message -->
+          <mj-text>
+            {% blocktrans %}
+            Your recording of "{{room_name}}" on {{recording_date}} at {{recording_time}} is now ready to download.
+            {% endblocktrans %}
+          </mj-text>
+          <mj-text>
+            <p>{% trans "To keep this recording permanently:" %}</p>
+            <ol>
+              <li>{% trans "Click the \"Open\" button below" %}</li>
+              <li>{% trans "Use the \"Download\" button in the interface" %}</li>
+              <li>{% trans "Save the file to your preferred location" %}</li>
+            </ol>
+          </mj-text>
+          <mj-button
+            href="{{link}}"
+            background-color="#000091"
+            color="white"
+            padding-bottom="30px"
+          >
+            {% trans "Open"%}
+          </mj-button>
+          <mj-text>
+            {% blocktrans %}
+            If you have any questions or need assistance, please contact our support team at {{support_email}}.
+            {% endblocktrans %}
+          </mj-text>
+          <mj-divider
+            border-width="1px"
+            border-style="solid"
+            border-color="#DDDDDD"
+            width="30%"
+            align="center"
+          />
+          <!-- Signature -->
+          <mj-text>
+            <p>
+              {% blocktrans %}
+              Thank you for using {{brandname}}.
+              {% endblocktrans %}
+            </p>
+          </mj-text>
+        </mj-column>
+      </mj-section>
+    </mj-wrapper>
+  </mj-body>
+</mjml>