appview/db: fix orphaned pipelines and triggers on repo deletion
pipelines and triggers were not cleaned up when a repository was
deleted. add repo_at foreign key with on delete cascade to both tables
so the database handles cleanup automatically.
pipeline_statuses also cascade through pipelines, ensuring the full
chain is removed. replaces repo_owner/repo_name lookups with repo_at
Signed-off-by: moshyfawn <email@moshyfawn.dev>