Feat/campaign export serializer 273

[eleven-smg]

Closes #273

Adds a campaign snapshot export serializer to the demo admin dashboard so a campaign (its metadata plus draft messages) can be exported as stable, versioned JSON that is easy to review and re-import.

Included

• types/campaignExport.ts — CampaignSnapshotExport envelope and CAMPAIGN_EXPORT_SCHEMA_VERSION.
• helpers/campaignExport.ts — buildCampaignExport, serializeCampaignSnapshot, buildCampaignExportFilename.
• tests/campaignExport.test.ts — unit tests for stable ordering, tag de-duplication, version metadata, determinism, and the filename.
• docs/CAMPAIGN_JSON_EXPORT.md — short schema and usage notes.
• index.ts — re-exports the new helpers and types from the feature barrel.

Behavior

• Drafts are sorted by id and reduced to known fields only.
• Tags are de-duplicated and sorted.
• Version metadata is included; the payload has no timestamps, so output is deterministic (the export date lives in the filename).

Scope

• All changes live under src/features/demo-admin-dashboard/. No files outside the folder are touched, the data stays fake and demo-only, and nothing is wired into production mail flows.