fix: close the gaps a security review found in the default install

Reported against 2.2.1 by someone reading the source. Every finding held.

**Sign-in had no ceiling.** Failures were logged with the caller's address
and nothing more. Two counters now — one per address, one per account —
because the two attacks look different: one source working through many
accounts is caught by the first, many sources working on one account by the
second, and behind a proxy only the second still means anything.

The count is kept in the process that serves the request. PLANKA needs no
Redis and the stock deployment is one container; run several and each keeps
its own count, which multiplies the ceiling by their number. That trade is
written where the limits are configured.

**The second factor could be guessed at leisure.** Six digits, and a
pending token that stayed valid for its full ten minutes however many codes
were wrong. Wrong codes are now counted on the session row — in the
database, so the count survives a restart and holds across every process —
and when the budget is spent the session is destroyed. After that even the
right code is refused and the login starts over from the password.

**Avatars, background images and favicons** checked the token's signature
and nothing else, so a revoked session, a deactivated account or a changed
password all kept working there for as long as the signature lasted, which
is a year by default. The five checks the API makes now live in one helper
that both use, rather than the shortened copy that had drifted from it.

**A link attachment's favicon** was fetched from wherever the URL pointed.
Storing a link is harmless — it is a string the user typed — but fetching
its icon is a request the server makes to an address the user chose, and
whether an icon came back reported on what is reachable from inside the
network. Server-side fetches now refuse private, loopback and link-local
addresses, `169.254.169.254` among them. The attachment is still created:
linking to an internal wiki is a legitimate thing to do, and it was the
server's own request that had to stop.

**The signing key.** Our own compose file ships `notsecretkey`, and it is
printed in the documentation — so on any instance that copied it, anyone can
sign a token for any account. PLANKA now says so on every start, and keeps
saying it, along with a key that is missing or shorter than 32 characters.
The placeholder carries the warning inline, where it is copied from.

**The backup script** wrote password hashes, live sessions, TOTP secrets and
SMTP credentials to an unencrypted archive. `BACKUP_PASSPHRASE` now encrypts
it, and without one the script says what it just put on disk. It also says
what it is — an example for the stock compose stack, not a backup concept —
and names the window between the database dump and the file copy, which no
ordering closes.

server/api/helpers/utils/is-safe-remote-url.js

@@ -0,0 +1,107 @@
+/*!
+ * Copyright (c) 2026 PLANKA Software GmbH
+ * Licensed under the Fair Use License: https://github.com/plankanban/planka/blob/master/LICENSE.md
+ */
+
+// SSRF guard for server-side fetches of user-supplied URLs (link previews).
+// Rejects anything but http/https and any host that resolves to a private,
+// loopback, link-local or otherwise-internal address — so a pasted
+// `http://169.254.169.254/…` or `http://localhost:1337` can't reach the
+// internal network. When an outgoing proxy is configured, egress policy is the
+// proxy's responsibility and we defer to it.
+
+const net = require('net');
+const dns = require('dns').promises;
+const { URL } = require('url');
+
+const isPrivateIPv4 = (ip) => {
+  const parts = ip.split('.').map((octet) => parseInt(octet, 10));
+  if (parts.length !== 4 || parts.some((octet) => Number.isNaN(octet))) {
+    return true;
+  }
+  const [a, b] = parts;
+  return (
+    a === 0 || // "this" network
+    a === 10 ||
+    a === 127 || // loopback
+    (a === 100 && b >= 64 && b <= 127) || // CGNAT 100.64.0.0/10
+    (a === 169 && b === 254) || // link-local (cloud metadata)
+    (a === 172 && b >= 16 && b <= 31) || // 172.16.0.0/12
+    (a === 192 && b === 168)
+  );
+};
+
+const LOCAL_IPV6_PREFIXES = ['fe8', 'fe9', 'fea', 'feb', 'fc', 'fd'];
+
+const isPrivateIPv6 = (ip) => {
+  const address = ip.toLowerCase();
+  if (address === '::1' || address === '::') {
+    return true;
+  }
+  // Link-local (fe80::/10) and unique-local (fc00::/7 → fc/fd prefixes).
+  if (LOCAL_IPV6_PREFIXES.some((prefix) => address.startsWith(prefix))) {
+    return true;
+  }
+  // IPv4-mapped (::ffff:a.b.c.d) — check the embedded v4.
+  const mapped = address.match(/::ffff:(\d+\.\d+\.\d+\.\d+)$/);
+  if (mapped) {
+    return isPrivateIPv4(mapped[1]);
+  }
+  return false;
+};
+
+const isPrivateAddress = (ip) => {
+  const family = net.isIP(ip);
+  if (family === 4) {
+    return isPrivateIPv4(ip);
+  }
+  if (family === 6) {
+    return isPrivateIPv6(ip);
+  }
+  return true; // not a recognizable IP → treat as unsafe
+};
+
+module.exports = {
+  inputs: {
+    url: {
+      type: 'string',
+      required: true,
+    },
+  },
+
+  async fn(inputs) {
+    let parsed;
+    try {
+      parsed = new URL(inputs.url);
+    } catch (error) {
+      return false;
+    }
+
+    if (parsed.protocol !== 'http:' && parsed.protocol !== 'https:') {
+      return false;
+    }
+
+    // Egress through a trusted proxy → let the proxy enforce destination policy.
+    if (sails.config.custom.outgoingProxy) {
+      return true;
+    }
+
+    const host = parsed.hostname.toLowerCase();
+    if (host === 'localhost' || host.endsWith('.localhost')) {
+      return false;
+    }
+
+    if (net.isIP(host)) {
+      return !isPrivateAddress(host);
+    }
+
+    let addresses;
+    try {
+      addresses = await dns.lookup(host, { all: true });
+    } catch (error) {
+      return false;
+    }
+
+    return addresses.length > 0 && addresses.every(({ address }) => !isPrivateAddress(address));
+  },
+};