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.

internal/store/model.go

@@ -59,13 +59,22 @@ const (
 	// StrategyManual rejects the stale push and leaves both copies in place
 	// for a human to reconcile.
 	StrategyManual Strategy = "manual"
+	// StrategyMerge decrypts both copies in memory and folds them together,
+	// so neither edit is lost. It is the only strategy that requires the
+	// server to hold the vault's key.
+	StrategyMerge Strategy = "merge"
 )
 
 // Valid reports whether s is a strategy that may be stored.
 func (s Strategy) Valid() bool {
-	return s == StrategyServerWins || s == StrategyClientWins || s == StrategyManual
+	return s == StrategyServerWins || s == StrategyClientWins ||
+		s == StrategyManual || s == StrategyMerge
 }
 
+// NeedsKey reports whether this strategy can only run when the server holds
+// the vault's key.
+func (s Strategy) NeedsKey() bool { return s == StrategyMerge }
+
 type User struct {
 	ID            string    `json:"id"`
 	Username      string    `json:"username"`
@@ -94,15 +103,19 @@ type Membership struct {
 }
 
 type Database struct {
-	ID            string    `json:"id"`
-	Name          string    `json:"name"`
-	OwnerGroupID  string    `json:"owner_group_id"`
-	MergeStrategy Strategy  `json:"merge_strategy"`
-	Version       int64     `json:"version"`
-	SHA256        string    `json:"sha256"`
-	Size          int64     `json:"size"`
-	UpdatedAt     time.Time `json:"updated_at"`
-	UpdatedBy     string    `json:"updated_by,omitempty"`
+	ID            string   `json:"id"`
+	Name          string   `json:"name"`
+	OwnerGroupID  string   `json:"owner_group_id"`
+	MergeStrategy Strategy `json:"merge_strategy"`
+	// WrappedKey is the vault's own password, encrypted under the server's
+	// master key. It is present only for vaults that are merged here, and it
+	// is never returned by the API.
+	WrappedKey string    `json:"wrapped_key,omitempty"`
+	Version    int64     `json:"version"`
+	SHA256     string    `json:"sha256"`
+	Size       int64     `json:"size"`
+	UpdatedAt  time.Time `json:"updated_at"`
+	UpdatedBy  string    `json:"updated_by,omitempty"`
 }
 
 type Grant struct {
@@ -128,9 +141,11 @@ func (i *Invite) Usable(now time.Time) bool {
 }
 
 type Session struct {
-	Token     string    `json:"token"`
-	UserID    string    `json:"user_id"`
-	Label     string    `json:"label,omitempty"`
+	Token  string `json:"token"`
+	UserID string `json:"user_id"`
+	Label  string `json:"label,omitempty"`
+	// CSRF guards the browser forms of a session held in a cookie.
+	CSRF      string    `json:"csrf,omitempty"`
 	CreatedAt time.Time `json:"created_at"`
 	ExpiresAt time.Time `json:"expires_at"`
 }