feat: add owner-gated community hard delete

[kalvinnchau]

Summary

• add DELETE /operator/communities/{host}?owner_pubkey=... with operator-signed NIP-98 authentication and owner attestation
• lock the current owner row and community before permanently deleting the tenant
• migrate all 22 parent-table community foreign keys to ON DELETE CASCADE; PostgreSQL propagates the policy to 14 partition constraint clones
• preserve the full hostname uniqueness index so deleted hosts are immediately reusable without mixed-version or rollback hazards
• bring the checked-in final-state schema.sql snapshot in sync with migration 0002 by including the previously omitted git_repo_names table; this is snapshot correction, not new runtime DDL

Safety

• non-matching owners receive a non-enumerating 404 and cannot delete
• migration integration test asserts every direct community FK, including partition clones, has CASCADE behavior
• hard-delete integration test verifies child rows are removed and recreation receives a fresh community UUID

Testing

• cargo fmt --all -- --check
• cargo clippy -p buzz-db -p buzz-relay --all-targets -- -D warnings
• focused migration embedding + real-Postgres fresh migration test
• real-Postgres hard-delete cascade/owner test
• real-Postgres operator API delete test
• pre-commit hooks
• pre-push hooks (Rust, desktop, Tauri, and mobile suites)