bridgev2: relay Matrix messages and reactions through Discord

[keithah]

Fix Matrix relay messages in bridgev2 Discord rooms so they are not sent as the relay account.

Relayed Matrix messages now go out through a Discord webhook for the portal. The webhook send uses the Matrix sender's display name, and when possible reuses the matching Discord ghost profile/avatar. It also handles the webhook-specific cases that normal channel sends don't cover: edits/deletes, media uploads, stale webhook recreation, safe allowed mentions, and a small reply fallback embed.

Incoming Discord echoes from the portal's own relay webhook are ignored so Matrix-originated webhook sends are not bridged back as new Discord-originated messages. This matches the old bridge invariant that relay webhook sends are tracked as Matrix-originated messages, not re-imported from the gateway echo.

Relayed Matrix reactions are wired up on the Discord connector side and use the configured relay Discord session as native Discord reactions. Discord does not support per-reaction webhook profile overrides, so reactions must not be represented as webhook messages. bridgev2 only passes relayed reactions to connectors with mautrix/go#510:

mautrix/go#510

Until that lands, the webhook message relay parts of this PR work, but relayed reactions are still blocked in mautrix-go before Discord sees them.

Tested:

• go test ./pkg/... ./cmd/mautrix-discord
• deployed on my bridge during development and tested Matrix-to-Discord relay messages, webhook sender profiles, webhook edit/delete behavior, and relayed native reactions

No changelog update included.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for relaying Matrix reactions via a configured logged-in Discord user session when the reacting Matrix user isn’t logged into Discord.

Changes:

• Introduces relay_reactions_from config and upgrade support to select a relay Matrix user whose Discord session will be used.
• Updates reaction handling to fall back to the relay user when a Matrix sender has no Discord session (and webhooks are enabled).
• Improves redaction handling to remove reactions using the appropriate user session and return clearer status reasons.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
portal.go	Adds relay reaction user lookup and uses it for reaction create/remove flows when sender isn’t logged in.
example-config.yaml	Documents the new relay_reactions_from configuration option.
config/upgrade.go	Copies bridge.relay_reactions_from during config upgrades.
config/bridge.go	Adds RelayReactionsFrom to the bridge config struct.

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