chore(migrations): renumber 108_add_backup_paths to 109 to avoid upstream collision

upstream/beta independently shipped 108_seed_sl_email_template_translations.js
(Slovenian email template translations) using the migration number
this branch had already claimed for 108_add_backup_paths.js. Knex's
filename-based ordering would have caused both to attempt the slot
at merge time.

Renamed via `git mv` so file history is preserved. All five
references updated in lockstep:
  - backend/src/services/_backupPathsBoot.js (require + comments)
  - backend/src/services/backupService.js (LEGACY_BACKUP_PATHS comment)
  - 3 integration test files (require + "migration 108" prose)
  - migration's own header comment, with a paragraph explaining the
    rename so reviewers don't wonder why the number jumped

**No data-migration impact for installs that already ran the
108-named version** (Ralf's beta, primarily): the migration's body
is idempotent — createTable is guarded by `hasTable`, and the seed
uses `onConflict('path').ignore()`. So when 109 runs against an
install whose backup_paths table is already populated, both the
schema step and the seed step no-op cleanly. The orphaned
`108_add_backup_paths.js` row in the `migrations` tracking table
sits harmlessly alongside the new `109_add_backup_paths.js` row.

No data lost, no double-insert, no schema drift. Mechanical rename
ahead of the PR opening.

backend/src/services/backupService.js

@@ -419,7 +419,7 @@ async function scanDirectory(dirPath, fileList, basePath, excludePatterns = [])
 
 /**
  * Hard-coded fallback when `backup_paths` is missing/empty. Mirrors
- * the canonical seed in migration 108 — kept here as defense in depth
+ * the canonical seed in migration 109 — kept here as defense in depth
  * so the walker can never silently degrade to "no directories scanned"
  * because of a seed problem.
  *