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/deployment/topology-guide.md

@@ -6,24 +6,8 @@ ScadaBridge uses a hub-and-spoke architecture:
 - **Central Cluster**: Two-node active/standby Akka.NET cluster for management, UI, and coordination.
 - **Site Clusters**: Two-node active/standby Akka.NET clusters at each remote site for data collection and local processing.
 
-```
-                    ┌──────────────────────────┐
-                    │     Central Cluster       │
-                    │  ┌──────┐    ┌──────┐    │
-  Users ──────────► │  │Node A│◄──►│Node B│    │
-  (HTTPS/LB)        │  │Active│    │Stby  │    │
-                    │  └──┬───┘    └──┬───┘    │
-                    └─────┼───────────┼────────┘
-                          │           │
-              ┌───────────┼───────────┼───────────┐
-              │           │           │           │
-        ┌─────▼─────┐ ┌──▼──────┐ ┌──▼──────┐ ┌──▼──────┐
-        │  Site 01   │ │ Site 02 │ │ Site 03 │ │ Site N  │
-        │ ┌──┐ ┌──┐ │ │ ┌──┐┌──┐│ │ ┌──┐┌──┐│ │ ┌──┐┌──┐│
-        │ │A │ │B │ │ │ │A ││B ││ │ │A ││B ││ │ │A ││B ││
-        │ └──┘ └──┘ │ │ └──┘└──┘│ │ └──┘└──┘│ │ └──┘└──┘│
-        └───────────┘ └─────────┘ └─────────┘ └─────────┘
-```
+![topology-architecture-overview](diagrams/topology-architecture-overview.png)
+<!-- source: diagrams/topology-architecture-overview.drawio — edit, then re-export with export-drawio.sh -->
 
 ## Central Cluster Setup