docs: describe worked-state check for automatic DX cluster spots (#91)

Add the new spot condition to the DX Cluster Server manual pages and the
forwarding section of the Features page in German and English, record
the fix in the v1.50 changelog, and note the rule in PROJECT_CONTEXT.

Fixes #91

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/PROJECT_CONTEXT.md

@@ -113,6 +113,7 @@ CR/LF framing, XML framing, ports/transports, callsign normalization and frequen
 - Comments are padded or truncated to exactly 30 characters. Automatic AirScout comments retain the locator first and use the compact form `JO51HK AP 1m/100%;4m/75%`.
 - A DX callsign longer than 12 characters is rejected and logged rather than truncated.
 - Trigger conditions, QRG recognition and normalisation, login, keepalive, multi-client delivery and the local-only trust boundary remain separate from line formatting.
+- Automatic spots are suppressed for stations already worked on the band of the sender's known frequency (`MessageBusManagementThread.isAlreadyWorkedForDxClusterSpot`); when only the global Worked flag exists without any per-band flag (Simplelogfile source), the global flag counts, an unknown band keeps the previous behaviour, and manual spots from the map window and the settings test spot are not affected (#91).
 
 ### Logging and Worked-state persistence