Make swallowed message uniqueness failures observable

[Xare123]

Problem

Message.save() swallowed uniqueness violations and returned as if persistence succeeded, leaving failed saves invisible in diagnostics.

Change

Retain existing conflict behavior while recording structured, content-safe diagnostics and whether the message remained without a persisted ID.

Validation

• one-file observability change
• git diff --check passes
• focused analyzer reports only pre-existing import/style findings

Privacy and behavior

Message text is not logged. This does not change database schema, conflict resolution, or delete records.