Purge/canonical follow-ups deferred from #196: slug normalization, mirror-ingest serialization, cache-publication fencing #583

Deferred out of PR #196's review rounds (all verified real, all needing one coherent design pass rather than per-site patches):

Slug normalization (one decision, several sites)

• Lock keys normalize the owner slug but cascade predicates and stored repo values do not: a same-slug create and purge do not mutually exclude.
• The purge cascade matches only the shortened slug, so child rows stored under a full did:key: owner survive purge, and matching pending sync items stay eligible for dequeue.
• Any fix must pick one canonical storage/matching form across lock keys, cascade predicates, and get_repo selection, and handle existing rows (normalizing future writes alone leaves them behind).

Mirror-ingest / writer serialization

• create_bounty inserts do not take the normalized slug lock, so authorize-then-insert can straddle the purge tombstone update. The lock shape depends on the normalization decision above.

Cache-publication fencing

• A cache-miss acquire can download the archive before purge and publish it after purge removes the row, directory, and archive. Needs cross-process fencing for in-flight downloads.

Canonical policy on sibling survival

• Purging a private canonical repo can leave a public same-slug mirror selected by get_repo; the skip-when-sibling-exists guard's sibling lookup needs the same normalization so the canonical policy is preserved.