Implement ZIP archive packaging for attachment bundles

Use the `archive` pub.dev package (`Archive`, `ArchiveFile`, `ZipEncoder`) for ZIP creation. Do NOT load all files into memory first — stream each file by downloading it via its signed URL using `http.get` (or Dart's `HttpClient`) and add the resulting bytes to the archive one file at a time, then release the reference. For the directory structure, build the path string as `'${activity.date.year}-${activity.date.month.toString().padLeft(2,'0')}/${activityTypeSlug}/${resolvedFilename}'`. Conflict resolution: maintain a `Map` of seen paths; on collision, increment counter and reinsert.

For streaming upload to Supabase Storage, use `supabase.storage.from(bucket).uploadBinary(path, bytes, fileOptions: FileOptions(upsert: true))` — for very large archives consider chunking using `upload` with a `Stream` if the SDK supports it. The method signature should be `Future buildAndUploadZip(List manifest, String organisationId, String reportPeriodId)` returning the signed URL or null. Run this method inside a try/catch with an audit log write on failure.

Unit tests (flutter_test) must cover: (1) directory structure — given a manifest with activities on two different dates and two types, assert the ZIP contains correctly nested paths; (2) conflict resolution — two attachments with identical filenames in the same directory, assert second is renamed with `_2` suffix; (3) empty manifest — assert method returns null and no upload is attempted; (4) skip on download failure — mock one file download throwing an exception, assert ZIP is created with remaining files and warning is logged; (5) idempotency — calling the method twice with the same report period ID uploads with `upsert: true` and does not throw. Integration test should create a real ZIP from 3 small test files, upload to Supabase test bucket, and verify download via signed URL produces a valid archive. Test coverage for the ZIP packaging method must be at least 85%.