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/services/database/app_database.dart

@@ -24,6 +24,14 @@ class Missions extends Table {
   TextColumn get waypointsJson => text()();
   BoolColumn get isDirty => boolean().withDefault(const Constant(true))();
   TextColumn get serverId => text().nullable()();
+
+  /// Gecachtes Bodenhoehenprofil (Doku 3.9), an einen routeKeyForTerrain()
+  /// gebunden (siehe TerrainProfile) - null, solange fuer die aktuelle Route
+  /// noch keins abgerufen wurde. Persistiert statt nur im TerrainNotifier
+  /// im Speicher gehalten, damit ein erneutes Oeffnen derselben Mission
+  /// (oder ein Import auf einem anderen Geraet) nicht jedes Mal alle
+  /// AWS-Terrarium-Kacheln neu laden muss.
+  TextColumn get terrainProfileJson => text().nullable()();
 }
 
 /// Drohnen-Profile (Doku 3.5/7.3) - FC-Typ bewusst nicht Teil des Schemas,
@@ -102,7 +110,7 @@ class AppDatabase extends _$AppDatabase {
   AppDatabase.forTesting(super.executor);
 
   @override
-  int get schemaVersion => 6;
+  int get schemaVersion => 7;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -146,6 +154,9 @@ class AppDatabase extends _$AppDatabase {
               appSettingsTable.connectionWifiLastSsid,
             );
           }
+          if (from < 7) {
+            await m.addColumn(missions, missions.terrainProfileJson);
+          }
         },
       );