docs: render architecture & flow diagrams as draw.io charts

Replace ASCII-art diagrams across the README and docs/ with editable
.drawio sources plus exported PNGs, so the diagrams render clearly in
rendered markdown and can be maintained/regenerated instead of being
hand-edited as fragile text art. Non-diagram blocks (code, folder
trees, UI wireframes) were left as text.

docs/requirements/Component-NotificationOutbox.md

@@ -24,24 +24,8 @@ SMTP and HTTP delivery is blocking I/O. Delivery work runs on a **dedicated bloc
 
 ## End-to-End Flow
 
-```
-Site script: Notify.To("list").Send(subject, body)
-    │  generate NotificationId (GUID) locally; return it to the script immediately
-    ▼
-Site Store-and-Forward Engine  (notification category, target = central)
-    │  durably forwards to central via Central–Site Communication (ClusterClient);
-    │  buffers/retries if central is unreachable
-    ▼
-Central ingest: insert-if-not-exists on NotificationId → Notifications table (Pending)
-    │  ack the site → site S&F clears the message
-    ▼
-Central Notification Outbox actor (singleton, active central node)
-    │  polls due rows; resolves the list; delivers via the matching adapter
-    ├── success            → Delivered
-    ├── transient failure  → Retrying (schedule NextAttemptAt)
-    └── permanent failure
-        / retries exhausted → Parked
-```
+![notificationoutbox-flow](diagrams/notificationoutbox-flow.png)
+<!-- source: diagrams/notificationoutbox-flow.drawio — edit, then re-export with export-drawio.sh -->
 
 The site forwards only `(listName, subject, body)` plus provenance — recipient resolution happens at central, at delivery time. This keeps notification-list definitions in one place and removes the deploy-to-sites artifact entirely.