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-storage-sqlite/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "@shade/storage-sqlite",
+  "version": "0.1.0",
+  "type": "module",
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "dependencies": {
+    "@shade/core": "workspace:*",
+    "@shade/crypto-web": "workspace:*",
+    "@shade/server": "workspace:*"
+  }
+}