fix(flatfile): harden flat-file backend against injection and traversal attacks

Security fixes for 7 vulnerabilities in database_flatfile.c:

1. Path traversal via crafted JID (HIGH):
   _ff_jid_to_dir() now strips '/', '\' -> '_' and collapses '..' -> '__',
   preventing a malicious federated JID from escaping the log directory.

2. Log injection via unescaped message body (HIGH):
   Add _ff_escape_message()/_ff_unescape_message() -- escape \n, \r, \\
   in message text on write, unescape on read. Prevents remote contacts
   from injecting fake log lines with forged sender/timestamp/encryption.

3. Metadata field injection (HIGH):
   Add _ff_escape_meta_value()/_ff_unescape_meta_value() -- escape |, ],
   \\, \n, \r in stanza_id/archive_id/replace_id. Parser uses escape-aware
   _ff_find_unescaped_char() and _ff_split_meta() instead of strchr/strsplit.

4. Sender ": " parsing confusion (MEDIUM):
   Escape ": " -> "\: " in XMPP resource on write. Parser uses
   _ff_find_unescaped_colonspace() + _ff_unescape_sender_resource().

5. LMC correction chain cycle -> infinite loop (MEDIUM):
   Add visited hash-set and FF_MAX_LMC_DEPTH (100) limit to chain walker.

6. Unbounded getline() -> OOM (MEDIUM):
   Add FF_MAX_LINE_LEN (10 MB) cap in _ff_readline(); overlength lines
   are skipped with a warning. Replaces all char buf[8192]/fgets() sites
   with dynamic getline() + truncated-line detection at EOF.

7. Symlink attack + TOCTOU on file creation (MEDIUM):
   Replace fopen("a") with open(O_WRONLY|O_APPEND|O_CREAT|O_EXCL|O_NOFOLLOW)
   + fdopen() -- atomic new-file detection, symlink rejection via O_NOFOLLOW.
   _ff_ensure_dir() checks g_lstat() for symlinks. File permissions 0600
   set via open() mode, not post-hoc g_chmod().

Dockerfile.arch

@@ -1,9 +1,11 @@
-FROM archlinux
+FROM archlinux:latest
 
 ENV TERM=xterm
 ENV CC="ccache gcc"
 
-RUN pacman -Syu --noconfirm
+RUN pacman-key --refresh-keys
+RUN pacman -Syyu --noconfirm
+
 # reflector is optional - if it fails due to network issues, continue with default mirrorlist
 RUN pacman -S --needed --noconfirm reflector && \
     (reflector --latest 20 --protocol https --sort rate --save /etc/pacman.d/mirrorlist || true)