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/time.ts

@@ -0,0 +1,25 @@
+/** UTC ISO-8601 time helpers. See PLAN.md §3.3. */
+
+/** Current time as a UTC ISO-8601 string with Z suffix. */
+export function nowIso(): string {
+  return new Date().toISOString();
+}
+
+/** Parse an ISO timestamp to a Date, throwing on invalid input. */
+export function parseIso(value: string): Date {
+  const d = new Date(value);
+  if (Number.isNaN(d.getTime())) {
+    throw new Error(`Invalid ISO timestamp: ${value}`);
+  }
+  return d;
+}
+
+/** Format a Date as a UTC ISO-8601 string. */
+export function toIso(d: Date): string {
+  return d.toISOString();
+}
+
+/** Compact HH:MM label (UTC) for the autosave indicator. */
+export function clockLabel(d: Date = new Date()): string {
+  return d.toISOString().slice(11, 16);
+}