* fix(backend): return 409 on duplicate prompt name
Pre-check by name in CreatePrompt and rename paths and surface
ErrPromptAlreadyExists as 409 with the underlying error message.
Previously the SQLite UNIQUE constraint bubbled up as a 500 with
a generic body, so the frontend had nothing actionable to show.
* feat(web): toast on prompt save errors with e2e coverage
Wire useToast in the Prompts settings page so create/update/delete
failures surface a warning instead of being swallowed silently. Add
data-testid hooks plus Playwright specs covering duplicate-name
create, rename-to-existing, and the happy path.
* fix(backend): close TOCTOU + tighten prompt error handling
Address PR review:
- log client errors (4xx) at Info, reserve Error for unexpected 500s
- switch handler error mapping to errors.Is so wrapped sentinels still
resolve to the right HTTP status
- drop the unreachable existing==nil branch in assertNameAvailable
- translate SQLite UNIQUE constraint errors that slip through the
pre-check race into ErrPromptAlreadyExists so concurrent dup-name
inserts return 409 instead of 500
José Almeida
5223a1cd17