v3.42.9: Fix all timeout bugs and deadlocks

CRITICAL FIXES:
- Encryption detection false positive (IsBackupEncrypted returned true for ALL files)
- 12 cmd.Wait() deadlocks fixed with channel-based context handling
- TUI timeout bugs: 60s->10min for safety checks, 15s->60s for DB listing
- diagnose.go timeouts: 60s->5min for tar/pg_restore operations
- Panic recovery added to parallel backup/restore goroutines
- Variable shadowing fix in restore/engine.go

These bugs caused pg_dump backups to fail through TUI for months.

README.md

@@ -143,7 +143,7 @@ Backup Execution
 
   Backup created: cluster_20251128_092928.tar.gz
   Size: 22.5 GB (compressed)
-  Location: /u01/dba/dumps/
+  Location: /var/backups/postgres/
   Databases: 7
   Checksum: SHA-256 verified
 ```