feat: create shared-code workspace for common code

Create a new shared-code workspace containing common code shared between
web-app and email-worker. This reduces code duplication and ensures
consistency across workspaces.

Structure:
- Root package.json defines workspaces (web-app, email-worker, shared-code)
- shared-code contains db-types.ts and shareChecksum.ts
- Configured as internal npm package (@evidencija-rezija/shared-code)
- No build step required (TypeScript source consumed directly)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

shared-code/src/index.ts

@@ -0,0 +1,5 @@
+// Database types
+export * from './db-types';
+
+// Share checksum utilities
+export * from './shareChecksum';