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/sharing/mission_share_codec.dart

@@ -1,7 +1,9 @@
 import 'dart:convert';
 
 import '../../app_info.dart';
+import '../../domain/mission/terrain_math.dart';
 import '../../domain/waypoint/flat_waypoint_list.dart';
+import '../database/terrain_json_codec.dart';
 import '../database/waypoint_json_codec.dart';
 
 /// Missionsdaten fuer den Export - unabhaengig von der Drift-Zeilenklasse,
@@ -12,12 +14,18 @@ class MissionExportData {
     this.placeName,
     required this.createdAt,
     required this.waypoints,
+    this.terrainProfile,
   });
 
   final String name;
   final String? placeName;
   final DateTime createdAt;
   final List<Waypoint> waypoints;
+
+  /// Gecachtes Bodenhoehenprofil (Doku 3.9), falls fuer die Mission bereits
+  /// eins abgerufen wurde - wird mitexportiert, damit ein Import auf einem
+  /// anderen Geraet nicht erneut alle Kacheln laden muss.
+  final TerrainProfile? terrainProfile;
 }
 
 /// Baut die Export-Datei fuer eine Mission (Doku 3.6, analog
@@ -33,6 +41,8 @@ String buildMissionExportJson(MissionExportData mission) {
       'placeName': mission.placeName,
       'createdAt': mission.createdAt.toUtc().toIso8601String(),
       'waypoints': [for (final w in mission.waypoints) waypointToJson(w)],
+      if (mission.terrainProfile != null)
+        'terrainProfile': terrainProfileToJson(mission.terrainProfile!),
     },
   };
   return const JsonEncoder.withIndent('  ').convert(map);
@@ -51,6 +61,7 @@ class ParsedMissionImport {
     this.placeName,
     required this.waypoints,
     this.versionMismatch,
+    this.terrainProfile,
   });
 
   final String name;
@@ -61,6 +72,11 @@ class ParsedMissionImport {
   /// der Hauptversionsnummer unterscheidet - sonst null (analog der
   /// "Imported from a different app version"-Warnung im HTML-Demonstrator).
   final String? versionMismatch;
+
+  /// Mitexportiertes Bodenhoehenprofil (Doku 3.9), falls die Datei eins
+  /// enthaelt - null bei Dateien aus einer Version ohne dieses Feld oder
+  /// wenn die Quellmission noch keins hatte.
+  final TerrainProfile? terrainProfile;
 }
 
 /// Parst eine importierte Missionsdatei - wirft [FormatException] bei
@@ -100,10 +116,16 @@ ParsedMissionImport parseMissionImportJson(String jsonText) {
       ? appVersion
       : null;
 
+  final terrainProfileRaw = missionData['terrainProfile'];
+  final terrainProfile = terrainProfileRaw is Map<String, dynamic>
+      ? terrainProfileFromJson(terrainProfileRaw)
+      : null;
+
   return ParsedMissionImport(
     name: name,
     placeName: placeName,
     waypoints: waypoints,
     versionMismatch: mismatch,
+    terrainProfile: terrainProfile,
   );
 }