feat(data): persist ndot and protect manually imported transceivers

- DB v2 migration: entries.ndot column so hasDecayed() actually works after
  reload (parsed ndot was dropped on write, making the decay detection dead
  code); radios.isCustom column to mark manually imported transceivers.
- updateTransceiversFromFile marks imported radios isCustom=true; remote
  updates now deleteManagedRadios() then insert the fresh snapshot, keeping
  manual imports (no source can refresh them, so nothing would bring them back).
- Wire MIGRATION_1_2 into MainContainer (fallbackToDestructiveMigration=false
  would otherwise crash on upgrade).

core/domain/src/main/java/com/rtbishop/look4sat/core/domain/model/SatRadio.kt

@@ -32,5 +32,7 @@ data class SatRadio(
     @SerialName("uplink_high") val uplinkHigh: Long?,
     @SerialName("uplink_mode") val uplinkMode: String?,
     @SerialName("invert") val isInverted: Boolean,
-    @SerialName("norad_cat_id") val catnum: Int?
+    @SerialName("norad_cat_id") val catnum: Int?,
+    /** Set for manually imported transceivers, which remote updates must not replace. */
+    val isCustom: Boolean = false
 )

core/domain/src/main/java/com/rtbishop/look4sat/core/domain/source/ILocalSource.kt

@@ -31,6 +31,9 @@ interface ILocalSource {
     suspend fun getIdsWithModes(modes: List<String>): List<Int>
     suspend fun getRadiosTotal(): Int
     suspend fun getRadiosWithId(id: Int): List<SatRadio>
-    suspend fun insertRadios(radios: List<SatRadio>)
+    suspend fun insertRadios(radios: List<SatRadio>, isCustom: Boolean = false)
+
+    /** Delete all non-custom transceivers, keeping manually imported ones. */
+    suspend fun deleteManagedRadios()
     suspend fun deleteRadios()
 }