docs(gateway): update UML diagrams following review feedback

- Fix class diagram: use aggregation (o--) for BleakScanner and MQTTClient
- Split sequence diagram into separate files for clarity
- Add sequence_startup.puml for gateway initialization
- Add sequence_data_collection.puml with error handling
- Use collections for BLE broadcaster to reflect multiplicity
- Remove loop construct — gateway reacts to packets, ignores intervals
- Add discard of unknown UUID packets
- Add handling of invalid sensor values

doc_resources/gateway/sequence_data_collection.puml

@@ -0,0 +1,27 @@
+@startuml
+skinparam sequenceMessageAlign center
+
+collections "BLE broadcaster" as thingy
+participant "Gateway\n(Raspberry Pi)" as gw
+participant "MQTT Broker\n(RabbitMQ)" as broker
+participant "Database\n(InfluxDB)" as db
+
+== Data collection ==
+thingy -> gw : BLE advertising packet
+
+alt UUID does not match ef680100
+    gw -> gw : discard packet
+else UUID matches
+    gw -> gw : extract raw payload
+    gw -> gw : decode_payload()
+    
+    alt unknown key or value out of range
+        gw -> gw : discard invalid value\nlog warning
+    else valid payload
+        gw -> gw : add UTC timestamp
+        gw -> broker : publish JSON\n{gateway_id}/{mac}/update
+        broker -> db : store measurement
+    end
+end
+
+@enduml