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/confirmation.go

@@ -1,6 +1,7 @@
 package tui
 
 import (
+	"context"
 	"fmt"
 	"strings"
 
@@ -15,6 +16,7 @@ type ConfirmationModel struct {
 	config    *config.Config
 	logger    logger.Logger
 	parent    tea.Model
+	ctx       context.Context
 	title     string
 	message   string
 	cursor    int
@@ -75,7 +77,7 @@ func (m ConfirmationModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 					return m.onConfirm()
 				}
 				// Default: execute cluster backup for backward compatibility
-				executor := NewBackupExecution(m.config, m.logger, m.parent, "cluster", "", 0)
+				executor := NewBackupExecution(m.config, m.logger, m.parent, m.ctx, "cluster", "", 0)
 				return executor, executor.Init()
 			}
 			return m.parent, nil