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/terrain_json_codec.dart

@@ -0,0 +1,31 @@
+import 'dart:convert';
+
+import '../../domain/mission/terrain_math.dart';
+
+/// JSON-(De-)Serialisierung fuer [TerrainProfile] - gemeinsam genutzt von
+/// der Missions-Persistenz ([MissionRepository]) und dem Mission-Sharing
+/// ([MissionShareCodec]), analog waypoint_json_codec.dart.
+Map<String, dynamic> terrainProfileToJson(TerrainProfile profile) => {
+      'routeKey': profile.routeKey,
+      'points': [
+        for (final p in profile.points)
+          {'distanceM': p.distanceM, 'elevationRelM': p.elevationRelM},
+      ],
+    };
+
+TerrainProfile terrainProfileFromJson(Map<String, dynamic> json) => TerrainProfile(
+      routeKey: json['routeKey'] as String,
+      points: [
+        for (final entry in json['points'] as List)
+          TerrainSamplePoint(
+            distanceM: (entry['distanceM'] as num).toDouble(),
+            elevationRelM: (entry['elevationRelM'] as num?)?.toDouble(),
+          ),
+      ],
+    );
+
+String encodeTerrainProfile(TerrainProfile profile) =>
+    jsonEncode(terrainProfileToJson(profile));
+
+TerrainProfile decodeTerrainProfile(String json) =>
+    terrainProfileFromJson(jsonDecode(json) as Map<String, dynamic>);