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>

package.json

@@ -0,0 +1,25 @@
+{
+  "name": "evidencija-rezija-monorepo",
+  "version": "1.0.0",
+  "description": "Property management and utility bills tracking monorepo",
+  "private": true,
+  "workspaces": [
+    "web-app",
+    "email-worker",
+    "shared-code"
+  ],
+  "scripts": {
+    "install:all": "npm install",
+    "build:web-app": "npm run build --workspace=web-app",
+    "build:email-worker": "npm run build --workspace=email-worker",
+    "dev:web-app": "npm run dev --workspace=web-app",
+    "dev:email-worker": "npm run start --workspace=email-worker"
+  },
+  "keywords": [
+    "property-management",
+    "utility-bills",
+    "monorepo"
+  ],
+  "author": "",
+  "license": "ISC"
+}