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/federationapi/server.go

@@ -61,6 +61,11 @@ type ServerOptions struct {
 	// endpoints unregistered.
 	Media *mediaapi.API
 
+	// UserKeys answers queries and claims of this server's users'
+	// end-to-end keys, and takes in key update EDUs. Nil leaves the key
+	// endpoints unregistered.
+	UserKeys KeyService
+
 	Logger *slog.Logger
 	Now    func() time.Time
 }
@@ -75,6 +80,7 @@ type Server struct {
 	profiles   Profiles
 	store      storage.FederationStore
 	media      *mediaapi.API
+	keyService KeyService
 	log        *slog.Logger
 	now        func() time.Time
 }
@@ -102,6 +108,7 @@ func NewServer(opts ServerOptions) (*Server, error) {
 		profiles:   opts.Profiles,
 		store:      opts.Store,
 		media:      opts.Media,
+		keyService: opts.UserKeys,
 		log:        opts.Logger,
 		now:        opts.Now,
 	}, nil
@@ -152,6 +159,7 @@ func (s *Server) RegisterRoutes(r *httputil.Router) {
 	authed("POST "+v1+"/publicRooms", s.handlePublicRoomsPost)
 
 	s.registerMediaRoutes(authed)
+	s.registerKeyRoutes(authed)
 }
 
 // authenticated checks X-Matrix and the allow/deny lists before the
@@ -382,9 +390,11 @@ func (s *Server) receiveEDU(ctx context.Context, origin string, edu federation.E
 			s.log.Debug("ignored a to-device EDU", "origin", origin, "error", err)
 		}
 
+	case federation.EDUDeviceListUp, federation.EDUSigningKeys:
+		s.receiveKeyEDU(ctx, origin, edu)
+
 	default:
-		// Presence and device list updates belong to phases that have not
-		// landed (presence is off by default; device lists are Phase 7).
+		// Presence is off by default and not federated yet.
 		s.log.Debug("ignored an EDU", "origin", origin, "type", edu.Type)
 	}
 }