feat(db): add komoot activity id for import deduplication

Signed-off-by: Marcus Fihlon <marcus@fihlon.swiss>

src/main/resources/db/migration/V32__add_komoot_activity_id_to_activities.sql

@@ -0,0 +1,15 @@
+-- Add optional internal reference to the originating Komoot activity.
+--
+-- This field is only used for import matching and deduplication. It is not
+-- intended for public display or API exposure.
+
+ALTER TABLE activities
+    ADD COLUMN komoot_activity_id BIGINT;
+
+-- A Komoot activity may only be imported once per local user.
+CREATE UNIQUE INDEX idx_activities_user_komoot_activity_id
+    ON activities(user_id, komoot_activity_id)
+    WHERE komoot_activity_id IS NOT NULL;
+
+COMMENT ON COLUMN activities.komoot_activity_id IS
+    'Optional internal Komoot activity ID used for import matching and deduplication';