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/widgets/missions_drones_panel.dart

@@ -229,6 +229,7 @@ class _MissionsDronesPanelState extends ConsumerState<MissionsDronesPanel> {
       placeName: full.placeName,
       createdAt: full.createdAt,
       waypoints: full.waypoints,
+      terrainProfile: full.terrainProfile,
     ));
     await ref.read(sharingServiceProvider).shareJsonFile(
           json: json,
@@ -251,11 +252,16 @@ class _MissionsDronesPanelState extends ConsumerState<MissionsDronesPanel> {
       messenger.showSnackBar(SnackBar(content: Text(e.message)));
       return;
     }
-    await ref.read(missionRepositoryProvider).upsert(
+    final id = await ref.read(missionRepositoryProvider).upsert(
           name: parsed.name,
           placeName: parsed.placeName,
           waypoints: parsed.waypoints,
         );
+    if (parsed.terrainProfile != null) {
+      await ref
+          .read(missionRepositoryProvider)
+          .saveTerrainProfile(id, parsed.terrainProfile!);
+    }
     if (parsed.versionMismatch != null) {
       messenger.showSnackBar(SnackBar(
         content: Text(