feat(workspace): full server + sync implementation, all tests green

- solitaire_server: Axum auth, sync push/pull, leaderboard, daily
  challenge, account deletion, JWT middleware, rate limiting via
  tower_governor, SQLite migrations, health endpoint
- solitaire_server: expose build_test_router (no rate limiting) so
  integration tests work without a peer IP in oneshot requests
- solitaire_sync: SyncPayload, merge logic, shared API types
- solitaire_data: SyncProvider trait, LocalOnlyProvider,
  SolitaireServerClient, auth_tokens keyring integration, blanket
  Box<dyn SyncProvider> impl
- solitaire_data/settings: derive Default on SyncBackend (clippy fix)
- .sqlx/: offline query cache so server compiles without a live DB
- sqlx: removed non-existent "offline" feature flag
- keyring v2: fixed Entry::new() returning Result<Entry>
- sqlx 0.8: all SQLite TEXT columns wrapped in Option<T>
- Integration tests: max_connections(1) on in-memory pool so all
  connections share the same schema

All 191 tests pass; cargo clippy -D warnings clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.sqlx/query-720edc3b85eec90744179f2c73ed9798814b734a2f3d57405ef95772ae66a24d.json

@@ -0,0 +1,12 @@
+{
+  "db_name": "SQLite",
+  "query": "INSERT INTO sync_state (user_id, stats_json, achievements_json, progress_json, last_modified)\n           VALUES (?, ?, ?, ?, ?)\n           ON CONFLICT(user_id) DO UPDATE SET\n               stats_json        = excluded.stats_json,\n               achievements_json = excluded.achievements_json,\n               progress_json     = excluded.progress_json,\n               last_modified     = excluded.last_modified",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Right": 5
+    },
+    "nullable": []
+  },
+  "hash": "720edc3b85eec90744179f2c73ed9798814b734a2f3d57405ef95772ae66a24d"
+}