feat: persistent storage — SQLite backends for crash resilience

Shade sessions and keys now survive server crashes, container restarts,
and power outages via SQLite with WAL mode.

New packages:
- @shade/storage-sqlite: SQLiteStorage (StorageProvider) + SqlitePrekeyStore
  (PrekeyStore), both using bun:sqlite with auto-created tables and WAL mode
- Serialization layer in shade-core for SessionState/keys ↔ JSON/base64

Docker usage: mount volume at /data, set SHADE_DB_PATH=/data/shade-client.db
Prekey server auto-detects SHADE_PREKEY_DB_PATH for SQLite persistence

Includes crash recovery integration test: encrypt → close DB → reopen →
conversation continues seamlessly.

129 tests, 0 failures.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shade-core/src/index.ts

@@ -6,3 +6,4 @@ export * from './errors.js';
 export * from './x3dh.js';
 export * from './ratchet.js';
 export { ShadeSessionManager } from './session.js';
+export * from './serialization.js';