An SSE subscription with a nonexistent projectId (stale client tab, e2e fixture
page using projectId "fixture") surfaced the PG startup-factory construction
chain as a 500 on every poll, filling operator logs with alarming
"failed to construct TaskStore" errors. Map project-not-found to a clean 404,
matching the project routes' existing handling.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>