feat(api): add in-memory GameSessionStore

Add the session storage infrastructure for tracking correct answers
during a game. Designed for easy swap to Valkey in Phase 4.

- GameSessionStore interface with create/get/delete methods, all async
  to match the eventual Valkey implementation
- InMemoryGameSessionStore backed by a Map
- GameSessionData holds only the answer key (questionId → correctOptionId)
- Also fix root build script to build packages in dependency order

apps/api/tsconfig.json

@@ -2,7 +2,7 @@
   "extends": "../../tsconfig.base.json",
   "references": [
     { "path": "../../packages/shared" },
-    { "path": "../../packages/db" }
+    { "path": "../../packages/db" },
   ],
   "compilerOptions": {
     "module": "NodeNext",
@@ -10,7 +10,7 @@
     "outDir": "./dist",
     "resolveJsonModule": true,
     "rootDir": ".",
-    "types": ["vitest/globals"]
+    "types": ["vitest/globals"],
   },
-  "include": ["src", "vitest.config.ts", "../../packages/db/src/models"]
+  "include": ["src", "vitest.config.ts"],
 }