Implement CHUNK #2: server layer, core schemas/adapters, contract tests, seed

Complete the server-backed storage layer for PubScript so the app can run
against a real Fastify + SQLite backend instead of only local IndexedDB.

Server (@pubscript/server):
- db.ts: better-sqlite3 schema + migrations (users, shares, projects, books,
  chapters, entities, boards, ai_sessions, audit); WAL mode; idempotent
  admin bootstrap via PUBSCRIPT_ADMIN_EMAIL/PASSWORD.
- auth.ts: argon2 hashing, JWT-ish session tokens, guards, auth routes.
- routes.ts: REST CRUD for projects/books/chapters/entities/boards/ai_sessions
  with multi-tenant isolation and optimistic concurrency (baseRev).
- ws.ts: Fastify websocket hub for autosave + presence events.
- mapping.ts: row <-> core type conversion.

Core (@pubscript/core):
- Zod domain schemas (projects, books, chapters, entities, boards, ai_sessions)
  exported from index.
- storage-local.ts (IndexedDB/Dexie) and storage-server.ts (REST) adapters
  implementing the StorageAdapter contract.

Tests & tooling:
- storage.contract.test.ts: parameterized over both adapters; wired as
  `pnpm contract` and added to CI.
- seed.ts (`pnpm --filter server seed`): creates demo project "The Glass Harbor"
  — 3 chapters of prose, 4 linked characters, 1 world location, 6-node board.
- Dockerfile + compose wiring for the server runtime.

Acceptance: seed project opens with 3 chapters, 4 linked characters, 1 board.

packages/core/src/storage.ts

@@ -0,0 +1,42 @@
+/**
+ * The StorageAdapter contract — the seam that makes web / desktop / local modes
+ * possible. Defined once here; every feature codes against this, never a backend.
+ * See PLAN.md §3.4.
+ */
+
+import type {
+  Board,
+  Chapter,
+  DocumentRef,
+  Entity,
+  NewProject,
+  Project,
+  ProjectMeta,
+  AISession,
+} from "./types.js";
+
+/** Live-update events fanned out to subscribed clients. */
+export type StoreEvent =
+  | { type: "doc-changed"; projectId: string; docId: string; rev: number }
+  | { type: "entity-changed"; projectId: string; entityId: string; kind: string }
+  | { type: "board-changed"; projectId: string; boardId: string }
+  | { type: "ai-session-changed"; projectId: string; sessionId: string }
+  | { type: "presence"; projectId: string; users: unknown[] }
+  | { type: "conflict"; docId: string; remoteRev: number };
+
+export interface StorageAdapter {
+  // projects
+  listProjects(): Promise<ProjectMeta[]>;
+  createProject(p: NewProject): Promise<Project>;
+  getProject(id: string): Promise<Project>;
+
+  // content (debounced autosave)
+  saveDocument(doc: DocumentRef): Promise<void>;
+  getChapter(id: string): Promise<Chapter | null>;
+  saveEntity(e: Entity): Promise<void>;
+  saveBoard(b: Board): Promise<void>;
+  saveAISession(s: AISession): Promise<void>;
+
+  // live updates
+  onEvent(cb: (e: StoreEvent) => void): () => void;
+}