[DwC export]: Create `extensions` join table

[grantfitzsimmons]

See #7745 for one part of this

Goal

Create the extensions join table that bridges exportdataset to schemamapping records used as extensions.

Background

An Export Package can have multiple extension mappings. The join table captures this one-to-many relationship and provides ordering for consistent cache table naming.

Schema

Field	Type	Notes
ExtensionID	Int (PK)	Auto-increment
ExportDataSetID	FK → exportdataset	Required
SchemaMappingID	FK → schemamapping	Required, Extension type only
SortOrder	Int	Determines ext1, ext2, … numbering

Acceptance Criteria

• Table created in migration with all fields above.
• Composite unique constraint on (ExportDataSetID, SchemaMappingID).
• SortOrder determines extension numbering in cache table names.
• Foreign key constraints enforced with CASCADE DELETE.
• ORM model created and registered.