Merge concurrent saves on the server, with vaults created from the web

Why: the point of a sync server is that two people can work at once. Picking
a winner was all this one could do while it could not read a vault.

What, end to end:
- A vault is created at /web or with `keeporg vault -create`: the server
  builds an empty KDBX 4 file, encrypts it with the password from the form,
  and stores it as version 1. An existing database can adopt a stored key
  with `keeporg db -id <id> -store-password`, which is verified against the
  stored file before it is accepted.
- A stale push to a `merge` vault is no longer refused. The server opens the
  stored version and the pushed one in memory, folds them together, and
  stores the re-encrypted result. Clients push as usual and pull what comes
  back. Any failure answers 409 merge_failed and the client keeps its copy
  aside, so a failed merge still loses nothing.
- At rest: vault contents stay KDBX-encrypted as always. A merge vault's
  password is kept AES-256-GCM-wrapped under a master key that lives outside
  the data directory, bound to the vault id, never returned by the API. A
  stolen data directory is ciphertext; a stolen data directory plus the
  master key is not. The server runs without a master key and simply refuses
  to create or merge vaults.

This moves the server inside the trust boundary for vaults it can merge,
which is inherent to merging and is stated plainly in the README, the
handover notes and the web form itself. Key-less strategies are unchanged and
remain the default for a database created without a password.

Proved: two clients editing the same version are merged rather than refused,
the newer edit of one entry wins, a merge failure leaves the stored vault
untouched, `merge` is refused without a stored key, the wrapped key never
appears in state.json or the API, and the web interface rejects a form
without its CSRF token, withholds its cookie over plain http, and only offers
groups the user administers. Checked by hand against the real thing: a vault
created in a browser, edited by keepassxc-cli on two sides from the same
version, merged by the server, and both entries read back out with
keepassxc-cli. make check, make test-race, android lint and both .deb builds
are clean.

packaging/deb/changelog

@@ -1,3 +1,19 @@
+keeporg (0.2.0) unstable; urgency=medium
+
+  * Server-side merging. A vault the server can open has its two concurrent
+    saves folded together in memory, by KeePass's own synchronization rules,
+    instead of one being refused. The losing version of an edited entry is
+    kept in that entry's history.
+  * New vault creation: a web interface at /web, and `keeporg vault -create`,
+    both of which build the KDBX file and set its password.
+  * Vault keys are stored encrypted under a master key in
+    /etc/keeporg/master.key, created on first start. Back it up separately
+    from /var/lib/keeporg.
+  * The KDBX 4 codec, its merge engine, and the ChaCha20, BLAKE2b and Argon2
+    it needs are part of the package; they carry no new dependencies.
+
+ -- tiredsysadmin <rcmd@garage.rcmd.space>  Thu, 29 Oct 2026 17:28:45 +0100
+
 keeporg (0.1.0) unstable; urgency=medium
 
   * First release.

packaging/deb/server/control.in

@@ -11,8 +11,14 @@ Description: KeePass database synchronization server
  writes between clients. It hosts many databases, controls access through a
  tree of user groups, and accepts registrations by invite link or OIDC.
  .
- It never opens a database and cannot read one: master passwords stay on the
- clients.
+ When two clients save at once it can merge both edits in memory, following
+ KeePass's own synchronization rules, so neither is lost.
+ .
+ Merging requires the server to hold the vault's password, which it keeps
+ encrypted under a master key in /etc/keeporg/master.key, created on first
+ start. Back that file up separately from /var/lib/keeporg: whoever holds
+ both can read every merge vault. Databases on any other conflict strategy
+ keep their key off the server entirely.
  .
  The service runs as the keeporg user with its data in /var/lib/keeporg and
  its configuration in /etc/keeporg/server.json. Terminate TLS in front of it