Pre-beta review: safer destructive commands and hardening

- backup:restore:full refuses on an installation with more than one user
  and asks before it starts (--force skips both, also to rerun after a
  failure halfway), and warns when the snapshot's database dump was not
  one consistent transaction.
- backup:user:restore --mode=replace asks before deleting an existing
  account; non-interactive use needs --force.
- The recovery kit file is created owner-only and never over an existing
  file or symlink; backup:key:import shows the deletion delay it read from
  the kit.
- Adding a location through the web API needs password confirmation, like
  removing one.
- Key derivation parameters from a kit or repository config are bounded,
  and a stalled network stream no longer spins at full CPU.
- Floating-point columns are dumped exactly instead of with 14 digits.

lib/Crypto/KeyRing.php

@@ -96,6 +96,12 @@ final class KeyRing {
 	/** @param array{v:int, salt:string, ops:int, mem:int, nonce:string, key:string} $wrapped */
 	public static function unwrap(array $wrapped, #[\SensitiveParameter] string $passphrase): self {
 		$b64 = static fn (string $s): string => sodium_base642bin($s, SODIUM_BASE64_VARIANT_ORIGINAL);
+		// ops/mem come from unauthenticated JSON (kit file, repository config): bound them, so a
+		// damaged or crafted file cannot make the KDF use unbounded memory or time.
+		if ($wrapped['ops'] < SODIUM_CRYPTO_PWHASH_OPSLIMIT_INTERACTIVE || $wrapped['ops'] > SODIUM_CRYPTO_PWHASH_OPSLIMIT_SENSITIVE
+			|| $wrapped['mem'] < SODIUM_CRYPTO_PWHASH_MEMLIMIT_INTERACTIVE || $wrapped['mem'] > SODIUM_CRYPTO_PWHASH_MEMLIMIT_SENSITIVE) {
+			throw new CryptoException('Unsupported key derivation parameters (damaged or foreign key file)');
+		}
 		$kek = sodium_crypto_pwhash(SODIUM_CRYPTO_SECRETBOX_KEYBYTES, $passphrase, $b64($wrapped['salt']),
 			$wrapped['ops'], $wrapped['mem'], SODIUM_CRYPTO_PWHASH_ALG_ARGON2ID13);
 		$master = sodium_crypto_secretbox_open($b64($wrapped['key']), $b64($wrapped['nonce']), $kek);