docs: add architecture diagram

docs/architecture.mmd

@@ -0,0 +1,9 @@
+flowchart LR
+  U[Browser\nNext.js] -->|HTTP JSON| B[Drogon API\nC++]
+  B -->|SQLite| DB[(csp.db)]
+
+  subgraph Backend Modules
+    C1[Controllers\n/auth, /health] --> S1[Services\nAuthService]
+    S1 --> R1[DB Layer\nSqliteDb + migrations]
+  end
+