Creating an episode or speaker profile with an existing name returns 500 #1451

Found by the pre-release smoke test (2026-10-04).

Problem

POST /api/episode-profiles and POST /api/speaker-profiles with a name that already exists return 500 (Failed to create episode profile). The unique indexes idx_episode_profile_name / idx_speaker_profile_name (migration 7) reject the insert, and the routers turn it into a generic 500. The client can't tell "name taken" from a server failure, and the UI shows a generic error.

API log:

Failed to create episode profile: Database index `idx_episode_profile_name` already contains 'smoke_portuguese', with record `episode_profile:...`
POST /api/episode-profiles HTTP/1.1" 500

Expected outcome

• Both create endpoints (and update, when renaming onto an existing name) return 409 (or 400) with a clear message, e.g. An episode profile named 'X' already exists. Prefer a typed exception from open_notebook.exceptions over a bare HTTPException.
• The profile form shows that message.

Acceptance criteria

• API tests: a duplicate name on create (and rename) returns 409/400 with the message, for episode and speaker profiles.
• pytest, ruff, mypy green.