docs(c4): add Email Service to L1 and L2 — NotificationService and PasswordResetService send SMTP

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

docs/architecture/c4-diagrams.md

@@ -14,9 +14,11 @@ C4Context
     Person(member, "Family Member", "Searches, browses, reads, and transcribes archived documents")
 
     System(familienarchiv, "Familienarchiv", "Web application for digitising, organising, and searching family documents")
+    System_Ext(mail, "Email Service", "SMTP server. Delivers notification emails (mentions, replies) and password-reset links.")
 
     Rel(admin, familienarchiv, "Manages via browser", "HTTPS")
     Rel(member, familienarchiv, "Searches, reads, and transcribes via browser", "HTTPS")
+    Rel(familienarchiv, mail, "Sends notification and password-reset emails (optional)", "SMTP")
 ```
 
 ---
@@ -30,6 +32,7 @@ C4Container
     title Container Diagram: Familienarchiv
 
     Person(user, "User", "Admin or family member")
+    System_Ext(mail, "Email Service", "SMTP server. Delivers notification and password-reset emails.")
 
     System_Boundary(archiv, "Familienarchiv (Docker Compose)") {
         Container(frontend, "Web Frontend", "SvelteKit / Node.js", "Server-side rendered UI. Handles auth session cookies, document search and viewer, transcription editor, annotation layer, family tree (Stammbaum), stories (Geschichten), activity feed (Chronik), enrichment workflow, and admin panel.")
@@ -51,6 +54,7 @@ C4Container
     Rel(backend, db, "Reads and writes metadata and sessions", "JDBC / SQL")
     Rel(backend, storage, "Uploads and streams document files", "HTTP / S3 API (AWS SDK v2)")
     Rel(backend, ocr, "OCR job requests with presigned MinIO URL", "HTTP / REST / JSON")
+    Rel(backend, mail, "Sends notification and password-reset emails (optional)", "SMTP")
     Rel(ocr, storage, "Fetches PDF via presigned URL", "HTTP / S3 presigned")
     Rel(mc, storage, "Creates bucket on startup", "MinIO Client CLI")
 ```