Delete seeded scenarios no seed file claims any more

`previous_titles` stops a rename stranding the row it left behind, but the rows
already stranded still had to be deleted by hand on every deployment. The
seeder now removes them on the next boot, which retires the stale "Road to the
Champion" demo without a database console.

Only rows with a NULL owner and `is_public` are considered, and a player's own
scenario is neither, so nothing anybody created is reachable. An adventure
started from a deleted demo survives: `adventures.scenario_id` is `ON DELETE
SET NULL`, and the adventure holds its own copies of the cards and scripts, so
it loses only the inherited cover art.

Two cases skip the sweep, because neither is an instruction to remove live
content: a seed file that fails to parse claims no title, and an empty seed
directory reads as a packaging failure.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PacdRuPXSkQQy4ZYdH32hF

README.md

@@ -202,7 +202,7 @@ development, Vite proxies `/api` to FastAPI.
 
 ## Tests
 
-539 backend tests: unit tests plus full HTTP integration through the real quickjs scripting
+549 backend tests: unit tests plus full HTTP integration through the real quickjs scripting
 engine, with the LLM provider mocked. CI runs them on every push, alongside the frontend
 lint/build and a Docker image build.