Add comprehensive process cleanup on TUI exit

- Created internal/cleanup package for orphaned process management
- KillOrphanedProcesses(): Finds and kills pg_dump, pg_restore, gzip, pigz
- killProcessGroup(): Kills entire process groups (handles pipelines)
- Pass parent context through all TUI operations (backup/restore inherit cancellation)
- Menu cancel now kills all child processes before exit
- Fixed context chain: menu.ctx → backup/restore operations
- No more zombie processes when user quits TUI mid-operation

Context chain:
- signal.NotifyContext in main.go → menu.ctx
- menu.ctx → backup_exec.ctx, restore_exec.ctx
- Child contexts inherit cancellation via context.WithTimeout(parentCtx)
- All exec.CommandContext use proper parent context

Prevents: Orphaned pg_dump/pg_restore eating CPU/disk after TUI quit

internal/tui/input.go

@@ -65,7 +65,7 @@ func (m InputModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 			// If this is from database selector, execute backup with ratio
 			if selector, ok := m.parent.(DatabaseSelectorModel); ok {
 				ratio, _ := strconv.Atoi(m.value)
-				executor := NewBackupExecution(selector.config, selector.logger, selector.parent, 
+				executor := NewBackupExecution(selector.config, selector.logger, selector.parent, selector.ctx,
 					selector.backupType, selector.selected, ratio)
 				return executor, executor.Init()
 			}