[OTA] Resolve the offered version per build environment (#2350)

checkForUpdates() reads an optional "envs" map from the update manifest and,
when it holds an entry for this build's ENV_NAME, offers that version instead
of the global latest_version. Boards with no entry are unaffected.

This lets the OTA server hold a specific product at an older release while the
rest of the fleet advances, with no per-board build flags. The resolved value
is republished as latest_version so the Home Assistant update entity compares
against it, and the map itself is dropped from the retained state topic to
keep it small.

Backward compatible both ways: old firmware ignores "envs", and a manifest
without "envs" behaves exactly as before.

OTA_JSON_URL is also guarded with #ifndef so a build environment can point the
update check at another OTA server. It was defined unconditionally, so a -D
build flag could not retarget it (the header definition always won). This is
needed to exercise the check against a test manifest, and is useful for
self-hosted OTA servers. No behaviour change without an override.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

main/User_config.h

@@ -251,10 +251,12 @@
 #  ifndef MQTT_HTTPS_FW_UPDATE_USE_PASSWORD
 #    define MQTT_HTTPS_FW_UPDATE_USE_PASSWORD 1 // Set this to 0 if not using TLS connection to MQTT broker to prevent clear text passwords being sent.
 #  endif
-#  if DEVELOPMENTOTA
-#    define OTA_JSON_URL "https://ota.openmqttgateway.com/binaries/dev/latest_version_dev.json" //OTA url used to discover new versions of the firmware from development nightly builds
-#  else
-#    define OTA_JSON_URL "https://ota.openmqttgateway.com/binaries/latest_version.json" //OTA url used to discover new versions of the firmware
+#  ifndef OTA_JSON_URL // may be overridden by a build flag to use another OTA server
+#    if DEVELOPMENTOTA
+#      define OTA_JSON_URL "https://ota.openmqttgateway.com/binaries/dev/latest_version_dev.json" //OTA url used to discover new versions of the firmware from development nightly builds
+#    else
+#      define OTA_JSON_URL "https://ota.openmqttgateway.com/binaries/latest_version.json" //OTA url used to discover new versions of the firmware
+#    endif
 #  endif
 #  define ENTITY_PICTURE   "https://github.com/1technophile/OpenMQTTGateway/raw/development/docs/img/Openmqttgateway_logo_mini_margins.png"
 #  define RELEASE_LINK_DEV "https://ota.openmqttgateway.com/binaries/dev/"