fix: emit chat.completed webhooks for successful API and streaming chats

[EvanProgramming]

Priority

P2 — registered webhooks only fire for the manual test endpoint.

Audit finding

_fire_webhooks() is implemented and /api/webhooks/test calls it, but neither /api/chat nor /api/chat/stream invokes it after a successful chat. A chat can therefore complete with no webhook notification despite the webhook API being configured.

Reproduction

With a registered chat.completed hook and _fire_webhooks replaced by a spy:

POST /api/chat              -> 200
events after chat           -> []
POST /api/webhooks/test     -> 200
events after test           -> [('test', ...)]

The real chat path completed; only the explicit test route produced an event.

Acceptance criteria

• Emit exactly one documented completion event after a successful non-streaming chat.
• Emit one event after the streaming generator reaches completion, with no event for an aborted/error stream.
• Include session/profile and a bounded reply/result or documented redacted payload.
• Webhook failures remain isolated from the user response and are logged/audited.
• Add endpoint tests for success, error, stream completion, and duplicate-event prevention.