Add generate-share-package endpoint for residence sharing

- Add POST /api/residences/:id/generate-share-package/ endpoint
- Add SharePackageResponse DTO with share code and metadata
- Add GenerateSharePackage service method to create one-time share codes
- Update JoinWithCode to deactivate share code after successful use

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

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

internal/dto/responses/residence.go

@@ -91,6 +91,15 @@ type GenerateShareCodeResponse struct {
 	ShareCode ShareCodeResponse `json:"share_code"`
 }
 
+// SharePackageResponse represents the response for generating a share package
+// This contains the share code plus metadata for the .casera file
+type SharePackageResponse struct {
+	ShareCode     string     `json:"share_code"`
+	ResidenceName string     `json:"residence_name"`
+	SharedBy      string     `json:"shared_by"`
+	ExpiresAt     *time.Time `json:"expires_at,omitempty"`
+}
+
 // === Factory Functions ===
 
 // NewResidenceUserResponse creates a ResidenceUserResponse from a User model