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/test/services/mission_repository_test.dart

@@ -1,6 +1,7 @@
 import 'package:drift/native.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import 'package:dmc_app/domain/mission/terrain_math.dart';
 import 'package:dmc_app/domain/waypoint/flat_waypoint_list.dart';
 import 'package:dmc_app/services/database/app_database.dart';
 import 'package:dmc_app/services/database/mission_repository.dart';
@@ -101,4 +102,52 @@ void main() {
     expect(loaded.waypoints.single.windDirFromDeg, 210);
     expect(loaded.waypoints.single.windElevationM, 2);
   });
+
+  group('Bodenhoehenprofil', () {
+    const profile = TerrainProfile(
+      routeKey: '52.00000,4.00000|52.01000,4.01000',
+      points: [
+        TerrainSamplePoint(distanceM: 0, elevationRelM: 0),
+        TerrainSamplePoint(distanceM: 30, elevationRelM: 2.5),
+        TerrainSamplePoint(distanceM: 60, elevationRelM: null),
+      ],
+    );
+
+    test('frisch angelegte Mission hat noch kein Profil', () async {
+      final id = await repository.upsert(name: 'No terrain', waypoints: waypoints);
+
+      final loaded = await repository.load(id);
+
+      expect(loaded!.terrainProfile, isNull);
+    });
+
+    test('saveTerrainProfile persistiert das Profil, load() gibt es zurueck', () async {
+      final id = await repository.upsert(name: 'With terrain', waypoints: waypoints);
+
+      await repository.saveTerrainProfile(id, profile);
+      final loaded = await repository.load(id);
+
+      expect(loaded!.terrainProfile, isNotNull);
+      expect(loaded.terrainProfile!.routeKey, profile.routeKey);
+      expect(loaded.terrainProfile!.points, hasLength(3));
+      expect(loaded.terrainProfile!.points[1].elevationRelM, 2.5);
+      expect(loaded.terrainProfile!.points[2].elevationRelM, isNull);
+    });
+
+    test('ein gewoehnlicher upsert (Wegpunkt-Autosave) loescht ein bereits '
+        'gespeichertes Profil nicht', () async {
+      final id = await repository.upsert(name: 'Autosave', waypoints: waypoints);
+      await repository.saveTerrainProfile(id, profile);
+
+      await repository.upsert(
+        id: id,
+        name: 'Autosave',
+        waypoints: [waypoints.first],
+      );
+
+      final loaded = await repository.load(id);
+      expect(loaded!.terrainProfile, isNotNull);
+      expect(loaded.terrainProfile!.routeKey, profile.routeKey);
+    });
+  });
 }