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/mission_repository.dart

@@ -2,8 +2,10 @@ import 'dart:convert';
 
 import 'package:drift/drift.dart';
 
+import '../../domain/mission/terrain_math.dart';
 import '../../domain/waypoint/flat_waypoint_list.dart';
 import 'app_database.dart';
+import 'terrain_json_codec.dart';
 import 'waypoint_json_codec.dart';
 
 /// Kopfzeilen-taugliche Zusammenfassung einer gespeicherten Mission, ohne
@@ -31,12 +33,18 @@ class LoadedMission {
     required 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 zuletzt
+  /// gespeicherte Route bereits eins abgerufen wurde - null, wenn noch
+  /// keins vorliegt oder es (Route inzwischen anders) verworfen werden muss.
+  final TerrainProfile? terrainProfile;
 }
 
 /// Persistenz-Zugriff fuer Missionen (Doku 4.15): kapselt die Drift-Tabelle
@@ -77,6 +85,9 @@ class MissionRepository {
       waypoints: [
         for (final entry in decoded) waypointFromJson(entry as Map<String, dynamic>),
       ],
+      terrainProfile: row.terrainProfileJson != null
+          ? decodeTerrainProfile(row.terrainProfileJson!)
+          : null,
     );
   }
 
@@ -110,6 +121,16 @@ class MissionRepository {
     return id;
   }
 
+  /// Persistiert ein frisch abgerufenes Bodenhoehenprofil fuer eine bereits
+  /// gespeicherte Mission (Doku 3.9) - separat von [upsert], damit ein
+  /// gewoehnlicher Wegpunkt-Autosave (bei dem noch kein Profil vorliegt)
+  /// nicht versehentlich ein zuvor gespeichertes Profil loescht.
+  Future<void> saveTerrainProfile(int id, TerrainProfile profile) async {
+    await (_db.update(_db.missions)..where((m) => m.id.equals(id))).write(
+      MissionsCompanion(terrainProfileJson: Value(encodeTerrainProfile(profile))),
+    );
+  }
+
   Future<void> rename(int id, String name) async {
     await (_db.update(_db.missions)..where((m) => m.id.equals(id))).write(
       MissionsCompanion(name: Value(name), updatedAt: Value(DateTime.now())),