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/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "@evidencija-rezija/shared-code",
+  "version": "1.0.0",
+  "description": "Shared code for web-app and email-worker",
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "scripts": {},
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "mongodb": "^6.0.0"
+  }
+}