Add slot invitation workflow

[zzawook]

Summary

• Adds a dedicated slot_invitations model and RPC workflow for creating, accepting, rejecting, and cancelling invitations.
• Treats pending invitations as reserved capacity in database checks, server actions, walk list cards, and walk detail pages.
• Adds volunteer invite search, in-app invitation response UI, invitation emails, and export/import table coverage.

Closes #95.

Testing

• npm run test (75 files, 941 tests)
• npm run test:coverage (branch coverage 81.11%, above the 80% threshold)
• git ls-files -m -o --exclude-standard -z | xargs -0 npx eslint --no-warn-ignored
• git diff --check
• NEXT_PUBLIC_SUPABASE_URL=http://127.0.0.1:54321 NEXT_PUBLIC_SUPABASE_ANON_KEY=dummy SUPABASE_SERVICE_ROLE_KEY=dummy NEXT_PUBLIC_APP_URL=http://localhost:3000 npm run build

Notes

• supabase db lint --local --schema public,private --fail-on warning could not run because the local Supabase Postgres container is not running on 127.0.0.1:54322.
• supabase db push --dry-run --linked could not run because this checkout is not linked to a Supabase project ref.

Ready for colleague review.