Support MySQL replication named channels (multi-source replication)

[renecannao]

MySQL Named Channels Support

Orchestrator currently assumes single-source replication (one MasterKey per replica). MySQL multi-source replication with named channels breaks this assumption at every layer.

Current failures

1. Discovery: only the last row from SHOW REPLICA STATUS survives
2. CHANGE REPLICATION SOURCE TO fails with error 3077 on multi-source replicas
3. STOP/START REPLICA affects all channels, not just the intended one
4. Topology graph misplaces multi-source replicas
5. Failover breaks other channels when promoting a multi-source replica

Implementation plan

Phase 1: Data model — ChannelStatus struct, database_instance_channels table, Instance struct extension

Phase 2: Discovery — Parse all SHOW REPLICA STATUS rows, store per-channel, choose canonical MasterKey

Phase 3: Channel-aware operations — All replication commands get FOR CHANNEL variants

Phase 4: Topology & visualization — Handle multi-parent nodes in topology graph and web UI

Phase 5: Failover — Channel-aware recovery that preserves non-managed channels

Phase 6: Testing — Functional tests with multi-source MySQL topology in CI

See detailed research in the implementation branch.