docs: sync latest doc updates to repo

docs/06-DATABASE.md

@@ -61,6 +61,67 @@ persistent storage. Data survives container restarts and rebuilds.
                  |--------------|
 ```
 
+```mermaid
+erDiagram
+    station {
+        int stationid
+        string name
+        string district
+        float lng
+        float lat
+        string mainriverbasin
+        string subriverbasin
+        float rainfall
+        float waterlevel
+        boolean siren
+        string cctv_link
+    }
+
+    rainfall {
+        int id
+        int stationid
+        datetime timestamp
+        float anncum
+        float daily
+        float hourly
+        float currentrf
+        float battery
+    }
+
+    waterlevel {
+        int id
+        int stationid
+        datetime datetime
+        float waterlevel
+        string alert
+        string warning
+        string danger
+    }
+
+    siren {
+        int id
+        int stationid
+        string stationtype
+        datetime active_time
+        string level
+    }
+
+    notification {
+        int id
+        int stationid
+        datetime timestamp
+        string stationtype
+        string level
+        datetime active_time
+    }
+
+    station ||--o{ rainfall : "has"
+    station ||--o{ waterlevel : "has"
+    station ||--o{ siren : "has"
+    station ||--o{ notification : "has"
+```
+
+
 **Note:** There are no database-level foreign keys between `station` and the
 data tables. Relationships are enforced at the application level by matching
 `stationid` values.