Add READMEs for remaining packages

- db-schema: Database schema types from kysely-codegen
- db: Database client and helper functions
- testing: Overview of testing packages
- test-helpers: Database testing utilities
- virtual-authenticator: WebAuthn virtual authenticator

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/db-schema/README.md

@@ -0,0 +1,24 @@
+# @reviq/db-schema
+
+Database schema types generated from PostgreSQL using kysely-codegen.
+
+## Usage
+
+```typescript
+import type { Database } from "@reviq/db-schema";
+```
+
+## Regenerating Types
+
+When the database schema changes, regenerate the types:
+
+```bash
+bun run --cwd packages/db-schema generate
+```
+
+This requires `DATABASE_URL` to be set and pointing to a database with the current schema.
+
+## Exports
+
+- `Database` - The full database type for use with Kysely
+- Table types for all database tables (e.g., `Users`, `Orgs`, `Sessions`)