M12b: category contract v2 on the server (schema 2, fourteen defaults, finer import mapping)

- migrations/0002_categories_v2.sql, byte-identical to contracts/schema/migrations/0002_categories_v2.sql
  (SPEC §14.1.1), with an identity test; SCHEMA_VERSION = 2 (E-1 reports it, the importer's check follows).
- Seed test on a fresh database (the fourteen rows of AC-7.2, server_version 11 to 14) and an upgrade test of a
  v1 database in use (0001 applied through the migrator, then users, a custom, a renamed and an archived default,
  an account, a recurring template, expenses): every earlier row unchanged, the four rows above the old
  sequence, foreign_key_check empty, schema_version 2, a pull from the old cursor returns exactly the four rows.
- DefaultCategoryKey with the fourteen keys (wire enum and contracts/openapi.yaml); icons 33 and colours 14
  (§9.7) in validation.
- The two category examples M12a held back (E-26 response-default, sync/pull-page-categories-v2) and its
  contracts-check patch for them and the enum.
- Importer: splitwise_categories.csv per the §10.8 v0.8 table (D-053), DEFAULT_KEYS of fourteen, synthetic
  fixtures for the finer mapping (Insurance/Försäkringar to insurance among them), a schema-1 refusal test.
- Test harnesses seed users at server_version 15/16 (after the fourteen seeds); pull counts 10 -> 14.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

server/crates/kvitt-server/tests/skeleton/routes.rs

@@ -104,7 +104,7 @@ async fn health_reports_ok_with_versions() {
     assert_eq!(reply.status, StatusCode::OK);
     assert_eq!(
         reply.json,
-        json!({"status": "ok", "server_version": env!("CARGO_PKG_VERSION"), "schema_version": 1, "db_ok": true})
+        json!({"status": "ok", "server_version": env!("CARGO_PKG_VERSION"), "schema_version": 2, "db_ok": true})
     );
     assert_eq!(
         reply.headers["x-kvitt-server-version"],
@@ -124,7 +124,7 @@ async fn health_is_503_degraded_when_the_database_is_gone() {
     assert_eq!(reply.status, StatusCode::SERVICE_UNAVAILABLE);
     assert_eq!(reply.json["status"], "degraded");
     assert_eq!(reply.json["db_ok"], false);
-    assert_eq!(reply.json["schema_version"], 1);
+    assert_eq!(reply.json["schema_version"], 2);
 }
 
 #[tokio::test]