docs(gateway): update diagrams following review feedback

- Simplify class diagram: remove KEY_* constants, add is_duplicate
  and on_disconnect, keep only behavior-relevant elements
- Update sequence diagram: replace repeated conditions with else,
  add deduplication step, update payload size to 14 bytes

doc_resources/gateway/sequence_data_collection.puml

@@ -11,7 +11,7 @@ thingy -> gw : BLE advertising packet
 
 alt company_id != 0xffff
     gw -> gw : discard packet
-else company_id == 0xffff
+else
     gw -> gw : check payload size
 
     alt payload size != 14 bytes
@@ -21,15 +21,19 @@ else company_id == 0xffff
 
         alt unknown key in payload
             gw -> gw : discard packet\nlog debug
-        else valid keys
+        else
             alt CO2 == 0xFFFFFFFF
                 gw -> gw : discard CO2 value\n(sensor not ready)
             end
 
-            gw -> gw : add UTC timestamp
-            gw -> broker : publish JSON\n{gateway_id}/{mac}/update
-            broker --> gw : on_publish confirmed
-            broker -> db : store measurement
+            alt duplicate within 10 seconds
+                gw -> gw : discard packet\n(deduplication)
+            else
+                gw -> gw : add UTC timestamp
+                gw -> broker : publish JSON\n{gateway_id}/{mac}/update
+                broker --> gw : on_publish confirmed
+                broker -> db : store measurement
+            end
         end
     end
 end