dbmeta: merge incoming metadata

By not implementing MetaMerger, UserLogin metadata such as
BridgedGuildIDs would get clobbered upon reauthing because the metadata
was replaced entirely. Implement CopyFrom so we can gain deeper control
over what is preserved upon reauth. Notably, preserve BridgedGuildIDs so
we can't get into a weird state where a guild is bridged but
simultaneously absent from BridgedGuildIDs, which would cause us to not
subscribe (OP 14) to it properly.

pkg/discordid/dbmeta.go

@@ -16,7 +16,10 @@
 
 package discordid
 
-import "github.com/bwmarrin/discordgo"
+import (
+	"github.com/bwmarrin/discordgo"
+	"maunium.net/go/mautrix/bridgev2/database"
+)
 
 type PortalMetadata struct {
 	// The ID of the Discord guild that the channel corresponding to this portal
@@ -32,3 +35,19 @@ type UserLoginMetadata struct {
 	HeartbeatSession discordgo.HeartbeatSession `json:"heartbeat_session"`
 	BridgedGuildIDs  map[string]bool            `json:"bridged_guild_ids,omitempty"`
 }
+
+var _ database.MetaMerger = (*UserLoginMetadata)(nil)
+
+func (ulm *UserLoginMetadata) CopyFrom(incoming any) {
+	incomingMeta, ok := incoming.(*UserLoginMetadata)
+	if !ok || incomingMeta == nil {
+		return
+	}
+
+	if incomingMeta.Token != "" {
+		ulm.Token = incomingMeta.Token
+	}
+	ulm.HeartbeatSession = discordgo.NewHeartbeatSession()
+
+	// Retain the BridgedGuildIDs from the existing login.
+}