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/terrain_provider.dart

@@ -1,8 +1,11 @@
+import 'dart:async';
+
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../../domain/mission/terrain_math.dart';
 import '../../domain/waypoint/flat_waypoint_list.dart';
 import '../../services/terrain/terrain_service.dart';
+import 'database_provider.dart';
 
 final terrainServiceProvider = Provider<TerrainService>((ref) => TerrainService());
 
@@ -30,7 +33,22 @@ class TerrainNotifier extends Notifier<TerrainState> {
   @override
   TerrainState build() => const TerrainState();
 
-  Future<void> ensureFor(List<Waypoint> waypoints) async {
+  /// Uebernimmt ein bereits vorliegendes Profil direkt, ohne einen
+  /// Netzwerkabruf auszuloesen - genutzt beim Laden einer gespeicherten
+  /// Mission (Doku 3.9: persistiertes Profil) bzw. zum Zuruecksetzen beim
+  /// Start/Wechsel einer Mission. ensureFor() prueft dessen routeKey wie
+  /// gewohnt, bevor es angezeigt wird, ein veraltetes Profil wird also nie
+  /// faelschlich fuer eine inzwischen andere Route benutzt.
+  void restore(TerrainProfile? profile) {
+    state = TerrainState(profile: profile);
+  }
+
+  /// [missionId] persistiert ein frisch abgerufenes Profil an der
+  /// gespeicherten Mission (Doku 3.9), damit ein erneutes Oeffnen derselben
+  /// Mission nicht wieder alle Kacheln neu laden muss - null, solange die
+  /// Mission noch nie gespeichert wurde (dann bleibt nur der In-Memory-Cache
+  /// dieser Notifier-Instanz).
+  Future<void> ensureFor(List<Waypoint> waypoints, {int? missionId}) async {
     if (waypoints.length < 2) {
       if (state.profile != null) state = const TerrainState();
       return;
@@ -48,6 +66,11 @@ class TerrainNotifier extends Notifier<TerrainState> {
       return;
     }
     state = TerrainState(profile: profile ?? state.profile, loading: false);
+    if (profile != null && missionId != null) {
+      unawaited(
+        ref.read(missionRepositoryProvider).saveTerrainProfile(missionId, profile),
+      );
+    }
   }
 }