Phase 7: E2EE support

Implements PLAN.md Phase 7: the server half of end-to-end encryption.
Devices upload identity, one-time and fallback keys; other devices query
and claim them, across servers too, and every one-time key is handed out
exactly once. Cross-signing, signature uploads, device list tracking and
server-side key backup complete the set.

internal/keyserver is the component. Keys are stored in canonical JSON
so that re-uploading the same key is a byte comparison, and signatures
uploaded later are kept apart from the key they sign and merged in on
read, so a key's own JSON stays what its owner uploaded. Device keys must
be for the caller's own device; cross-signing keys must be the
uploader's, carry the right usage and, for self- and user-signing keys,
be signed by the master key. Signatures are verified against the key as
stored. Replacing a master key asks for the password; a first upload or
an upload of the keys already stored does not, as the spec allows. A
signature of another user's master key is shown only to the user who
made it and never federated.

Every write that changes what /keys/query answers moves the device list
stream in the same transaction, one position per device, and wakes the
syncs of everyone sharing a room. The stream is the sixth field of a sync
token; older tokens still parse. The syncapi works out device_lists
changed and left from the stream and from membership moves in the
window, skipping invites and knocks, and reports
device_one_time_keys_count and device_unused_fallback_key_types, the
latter as [] when empty. GET /keys/changes uses the same logic.

Over federation: /user/keys/query, /user/keys/claim and
/user/devices/{userId}, and m.device_list_update and m.signing_key_update
both ways. Remote keys are not cached; an update about a remote user
sharing a room moves them in the stream so local clients re-query.
Device display names are not sent to other servers. With federation off,
remote users are reported in failures at once.

Deleting a device deletes its keys, through a new userapi
DeviceObserver. Key backup keeps the better copy of each session by the
spec's rule, never reuses a deleted version's number, and answers writes
to a stale version with M_WRONG_ROOM_KEYS_VERSION.

Storage is migration 0008 for sqlite and postgres, with a PocketBase
stub. The Complement target sets gain the key, device list, fallback,
cross-signing and backup tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/internal/httputil/errors.go

@@ -39,6 +39,10 @@ const (
 	// Media codes, added with the mediaapi in Phase 6.
 	ErrCodeNotYetUploaded  = "M_NOT_YET_UPLOADED"
 	ErrCodeCannotOverwrite = "M_CANNOT_OVERWRITE_MEDIA"
+
+	// End-to-end encryption codes, added with the keyserver in Phase 7.
+	ErrCodeInvalidSignature     = "M_INVALID_SIGNATURE"
+	ErrCodeWrongRoomKeysVersion = "M_WRONG_ROOM_KEYS_VERSION"
 )
 
 // MatrixError is the standard Matrix error envelope. Every non-2xx response
@@ -58,6 +62,10 @@ type MatrixError struct {
 	// encryption state and re-login.
 	SoftLogout bool `json:"soft_logout,omitempty"`
 
+	// CurrentVersion accompanies M_WRONG_ROOM_KEYS_VERSION: the backup
+	// version the client should be writing to instead.
+	CurrentVersion string `json:"current_version,omitempty"`
+
 	// status is the HTTP status to send; not part of the JSON body.
 	status int
 }