feat: wire SendGridEmailBackend with from_email/reply_to settings

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/email.py

@@ -8,12 +8,14 @@ intentionally left to be wired in R3-C.
 from __future__ import annotations
 
 import json
-import os
 import sys
 from datetime import datetime, timezone
 from pathlib import Path
 from typing import Optional, Protocol
 
+from sendgrid import SendGridAPIClient
+from sendgrid.helpers.mail import Mail, ReplyTo
+
 from app.config import settings
 
 
@@ -66,11 +68,10 @@ class ConsoleEmailBackend:
 
 
 class SendGridEmailBackend:
-    """Stub. Wire in R3-C (real SendGrid client + sandbox mode + retries).
-
-    Deliberately raises so a misconfigured prod env fails loudly instead of
-    silently dropping mail.
-    """
+    def __init__(self) -> None:
+        self._client = SendGridAPIClient(settings.SENDGRID_API_KEY)
+        self._from_email = settings.SENDGRID_FROM_EMAIL
+        self._reply_to = settings.SENDGRID_REPLY_TO
 
     def send(
         self,
@@ -78,8 +79,20 @@ class SendGridEmailBackend:
         subject: str,
         html: str,
         text: Optional[str] = None,
-    ) -> None:  # pragma: no cover - stub
-        raise NotImplementedError("Wire SendGrid in R3-C")
+    ) -> None:
+        message = Mail(
+            from_email=self._from_email,
+            to_emails=to,
+            subject=subject,
+            html_content=html,
+            plain_text_content=text,
+        )
+        message.reply_to = ReplyTo(self._reply_to)
+        response = self._client.send(message)
+        if response.status_code >= 400:
+            raise RuntimeError(
+                f"SendGrid error {response.status_code}: {response.body}"
+            )
 
 
 def get_email_backend() -> EmailBackend: