Condense WordPress backup cleanup report format

[locus313]

Problem

On servers with many domains, the report emailed/printed a repetitive 3-line
block per domain, even when nothing was removed for that domain:

Domain: example1.com
  Backups found: 3
  Would remove (dry-run): 0

Domain: example2.com
  Backups found: 1
  Would remove (dry-run): 0
...

This becomes an unreadable wall of text with 50+ domains.

Fix

build_report() now produces:

1. A one-line summary with totals (domains scanned, backups found, removed)
2. A "Backups removed by domain" section, listing only domains that actually
   had something removed/would-remove, with filenames indented underneath
3. A compact one-line-per-domain "Backups found by domain" table

Testing

• shellcheck clean (pre-existing intentional-glob info only)
• Manual test with 4 fake domains (mixed found/removed/zero counts, dry-run)
  confirms the summary, removed-section, and counts table render correctly