feat(backend): Implement email service infrastructure for Day 7

Add complete email service infrastructure with Mock and SMTP implementations.

Changes:
- Created EmailMessage domain model for email data
- Added IEmailService interface for email sending
- Implemented MockEmailService for development/testing (logs emails)
- Implemented SmtpEmailService for production SMTP sending
- Added IEmailTemplateService interface for email templates
- Implemented EmailTemplateService with HTML templates for verification, password reset, and invitation emails
- Registered email services in DependencyInjection with provider selection
- Added email configuration to appsettings.Development.json (Mock provider by default)

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

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

colaflow-api/src/ColaFlow.API/appsettings.Development.json

@@ -10,6 +10,18 @@
     "PMDatabase": "Host=localhost;Port=5432;Database=colaflow_pm;Username=colaflow;Password=colaflow_dev_password",
     "DefaultConnection": "Host=localhost;Port=5432;Database=colaflow_identity;Username=colaflow;Password=colaflow_dev_password"
   },
+  "Email": {
+    "Provider": "Mock",
+    "From": "noreply@colaflow.local",
+    "FromName": "ColaFlow",
+    "Smtp": {
+      "Host": "smtp.example.com",
+      "Port": "587",
+      "Username": "",
+      "Password": "",
+      "EnableSsl": "true"
+    }
+  },
   "MediatR": {
     "LicenseKey": "eyJhbGciOiJSUzI1NiIsImtpZCI6Ikx1Y2t5UGVubnlTb2Z0d2FyZUxpY2Vuc2VLZXkvYmJiMTNhY2I1OTkwNGQ4OWI0Y2IxYzg1ZjA4OGNjZjkiLCJ0eXAiOiJKV1QifQ.eyJpc3MiOiJodHRwczovL2x1Y2t5cGVubnlzb2Z0d2FyZS5jb20iLCJhdWQiOiJMdWNreVBlbm55U29mdHdhcmUiLCJleHAiOiIxNzkzNTc3NjAwIiwiaWF0IjoiMTc2MjEyNTU2MiIsImFjY291bnRfaWQiOiIwMTlhNDZkZGZiZjk3YTk4Yjg1ZTVmOTllNWRhZjIwNyIsImN1c3RvbWVyX2lkIjoiY3RtXzAxazkzZHdnOG0weDByanp3Mm5rM2dxeDExIiwic3ViX2lkIjoiLSIsImVkaXRpb24iOiIwIiwidHlwZSI6IjIifQ.V45vUlze27pQG3Vs9dvagyUTSp-a74ymB6I0TIGD_NwFt1mMMPsuVXOKH1qK7A7V5qDQBvYyryzJy8xRE1rRKq2MJKgyfYjvzuGkpBbKbM6JRQPYknb5tjF-Rf3LAeWp73FiqbPZOPt5saCsoKqUHej-4zcKg5GA4y-PpGaGAONKyqwK9G2rvc1BUHfEnHKRMr0pprA5W1Yx-Lry85KOckUsI043HGOdfbubnGdAZs74FKvrV2qVir6K6VsZjWwX8IFnl1CzxjICa5MxyHOAVpXRnRtMt6fpsA1fMstFuRjq_2sbqGfsTv6LyCzLPnXdmU5DnWZHUcjy0xlAT_f0aw"
   },