Persist the ground elevation profile with the mission

The Mini-Altitude-Profile terrain data only ever lived in TerrainNotifier's
in-memory state (one slot, keyed by route). Every app restart, or every
switch away from and back to a mission, forced a full re-fetch of all
AWS Terrarium elevation tiles for the route - the noticeably slow load
the user reported for some missions was this happening on every visit,
not just once.

Adds a nullable terrainProfileJson column to the missions table
(schema v6->v7) and a saveTerrainProfile() repository method, kept
separate from the regular waypoint upsert() so an ordinary autosave never
clobbers an already-cached profile. TerrainNotifier persists a profile
right after a successful fetch (fire-and-forget) and gains restore(),
called from CurrentMissionMetaNotifier whenever a mission is loaded/
started so a previously fetched profile is available immediately -
ensureFor() still validates its routeKey before use, so a stale restored
profile is never shown for a route that has since changed.

Also included in mission export/import (MissionExportData/
ParsedMissionImport) so sharing a mission carries its terrain cache along
instead of forcing the recipient to refetch it.

Sample-count/resolution stays as-is for now (still fixed 30m spacing,
10-2000 samples) - adapting the sampling density to terrain variance
(e.g. coarser sampling over flat ground) is a separate follow-up.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/ui/providers/mission_meta_provider.dart

@@ -7,6 +7,7 @@ import 'current_mission_provider.dart';
 import 'database_provider.dart';
 import 'follow_drone_provider.dart';
 import 'map_controller_provider.dart';
+import 'terrain_provider.dart';
 
 /// Metadaten der aktuell in Bearbeitung befindlichen Mission - ergaenzt
 /// currentMissionProvider (das nur die Wegpunkte haelt) um Name/Ort und die
@@ -58,6 +59,7 @@ class CurrentMissionMetaNotifier extends Notifier<MissionMeta> {
     ref.read(currentMissionProvider.notifier).replaceAll(loaded.waypoints);
     _suppressAutosave = false;
     state = MissionMeta(id: missionId, name: loaded.name, placeName: loaded.placeName);
+    ref.read(terrainProvider.notifier).restore(loaded.terrainProfile);
     _fitMapToWaypoints(loaded.waypoints);
   }
 
@@ -77,6 +79,7 @@ class CurrentMissionMetaNotifier extends Notifier<MissionMeta> {
     ref.read(currentMissionProvider.notifier).replaceAll(const []);
     _suppressAutosave = false;
     state = newState;
+    ref.read(terrainProvider.notifier).restore(null);
     await ref.read(appSettingsRepositoryProvider).setCurrentMissionId(null);
   }
 
@@ -89,6 +92,7 @@ class CurrentMissionMetaNotifier extends Notifier<MissionMeta> {
     ref.read(currentMissionProvider.notifier).replaceAll(loaded.waypoints);
     _suppressAutosave = false;
     state = MissionMeta(id: id, name: loaded.name, placeName: loaded.placeName);
+    ref.read(terrainProvider.notifier).restore(loaded.terrainProfile);
     _fitMapToWaypoints(loaded.waypoints);
     await ref.read(appSettingsRepositoryProvider).setCurrentMissionId(id);
   }