Skip empty transcript summarization using dedicated flag

[dembrane-sam-bot]

Instead of writing summary='empty' which has unintended UI consequences (such as showing 'empty' as the literal summary text to users), this PR introduces a dedicated has_empty_transcript boolean field on the conversation model.

When a finished conversation has an empty transcript, we flag has_empty_transcript = True in the DB and return early. The catch-up scheduler then excludes any conversations where has_empty_transcript == True.

[spashii]

Needs two things before it can land: (1) has_empty_transcript does not exist in the Directus snapshot (echo/directus/sync/snapshot/), so the write fails in every environment. It needs the field created via the migration-script + snapshot-pull flow in AGENTS.md. (2) Rebase onto main: #898 already landed the empty-string query filter fix, so this now conflicts in api/conversation.py and conversation_utils.py. The remaining value here is the dedicated flag that stops the ~5-minute catch-up scheduler from re-summarizing empty conversations forever, which is worth keeping.

[dembrane-sam-bot]

Done.

1. Rebased onto main to resolve the conflicts from fix(summary): correct Directus empty string query filter #898.
2. Added the has_empty_transcript field to the Directus schema snapshot (echo/directus/sync/snapshot/fields/conversation/has_empty_transcript.json) and created the idempotent migration script (echo/directus/migrations/add_has_empty_transcript_field.py).
3. Registered has_empty_transcript in the ConversationService.update schema and updated the catch-up scheduler query in conversation_utils.py to exclude has_empty_transcript == True.

Ready for review!