Support multiple AWS SES tenants (or multiple integrations) for reputation isolation per sender/brand #419

Problem

Currently a workspace can only have one active marketing provider and one active transactional provider.

When using Amazon SES, “SES tenant isolation” is scoped to the integration, not to individual senders.
This means that all senders defined inside the same SES integration share:

• The same SES tenant
• The same reputation metrics
• The same suppression list

If one sender (e.g. news@) has poor engagement or a high bounce/complaint rate, it negatively affects every other sender (partner@, support@, etc.) that lives in the same integration.

Why this is a problem

Many teams need to send from multiple brands or distinct email identities (newsletters, partner communications, transactional, etc.) from the same workspace.
They want these streams to have isolated reputation so that a problem with one stream does not hurt the others.

Desired solution

One (or both) of the following:

1. Allow multiple active integrations of the same type (especially SES) and let the user choose which integration/sender combination to use at send time (Broadcast, Automation, Transactional).

2. Per-sender tenant isolation
   When SES tenant isolation is enabled, allow Notifuse to create/use a separate SES tenant per sender (or per group of senders) instead of only one tenant per integration.

Benefits

• Proper reputation isolation between different sending identities
• Ability to use multiple AWS accounts or multiple tenants without constantly switching the active provider
• Cleaner multi-brand setups inside a single workspace

Current workaround

• Create separate workspaces (loses shared contacts/lists/templates), or
• Manually switch the active marketing/transactional provider before every send (very error-prone).

Both workarounds are impractical for production use.