fix(wifi): keep the legacy power-save default off; store explicit min as 3

Stored 0 was the shipped default from 2026-01-02 to 2026-03-28, and every
association path has run it with power save off since. Reading it as
MIN_MODEM, as F11 did, would have put every node set up in that window
into modem sleep on its next association. 0 now reads as `none`, and
`set wifi.powersave min` stores a new value 3. /mqtt.json accepts 0..3;
older firmware repairs 3 to `none` on load. Binary snapshots are no longer
written, so 3 never reaches a legacy layout.

src/helpers/MQTTPrefsSerializer.h

@@ -4,6 +4,7 @@
 #include <helpers/MQTTObserverValidation.h>
 #include <helpers/MQTTPresets.h>
 #include <helpers/MQTTPrefsStorage.h>
+#include <helpers/WifiPowerSavePolicy.h>
 
 #ifdef WITH_MQTT_BRIDGE
 
@@ -50,7 +51,7 @@ class MQTTPrefsSerializer : public ConfigSerializer {
   public:
     explicit WifiPrefs(MQTTPrefs* prefs) : _prefs(prefs), _power_save(prefs->wifi_power_save) {}
     bool apply(bool* repaired) {
-      if (_power_save < 0 || _power_save > 2) {
+      if (_power_save < 0 || _power_save > WifiPowerSavePolicy::kMaxStoredValue) {
         _power_save = 1;
         *repaired = true;
       }

src/helpers/WifiPowerSavePolicy.h

@@ -11,15 +11,18 @@
 // to `min` silently ran with power save off after its first reconnect while
 // `get wifi.powersave` still said min.
 //
-// Stored values are fleet state — never renumber them. The product default is
-// `none`, which is a *default* (MQTTDefaults.h), not a reinterpretation of an
-// operator's explicit `min`.
+// Stored values are fleet state — never renumber them. 0 was the shipped
+// default from 2026-01-02 to 2026-03-28, and every association path ran it with
+// power save off, so it keeps meaning `none`; an explicit `min` is stored as 3.
+// Older firmware repairs 3 to `none` when it loads /mqtt.json.
 namespace WifiPowerSavePolicy {
 
 enum StoredValue : uint8_t {
-  kMin  = 0,   // WIFI_PS_MIN_MODEM
-  kNone = 1,   // WIFI_PS_NONE  (default)
-  kMax  = 2,   // WIFI_PS_MAX_MODEM
+  kLegacyDefault = 0,   // WIFI_PS_NONE: the old default, never an operator choice
+  kNone = 1,            // WIFI_PS_NONE  (default)
+  kMax  = 2,            // WIFI_PS_MAX_MODEM
+  kMin  = 3,            // WIFI_PS_MIN_MODEM
+  kMaxStoredValue = kMin,
 };
 
 // Mirrors wifi_ps_type_t. MQTTBridge.cpp static_asserts these against the SDK.