fix: replace repo_sync_index wiring with purgatory announcement sync timer

Instead of threading repo_sync_index through PolicyContext/builder.rs/main.rs
to handle user-submitted purgatory announcements, add a simple background
timer (run_purgatory_announcement_sync, every 5s) that scans the purgatory
for announcement entries and registers them in repo_sync_index as StateOnly.

This is simpler and covers both flows:
- Sync-path announcements: inline registration still happens during event
  processing (sync/mod.rs:1839+), timer provides a safety net
- User-submitted announcements: SelfSubscriber never sees them (rejected
  from DB), timer is the primary registration path

The timer calls sync_purgatory_announcements_to_index() which:
1. Snapshots purgatory via new announcements_for_sync() public method
2. Or_inserts StateOnly entries (never downgrades Full entries)
3. Detects newly added relay URLs and calls handle_new_sync_filters to
   connect and subscribe - fixing the failing test that expected relay
   discovery from a user-submitted purgatory announcement

Removes: repo_sync_index field from PolicyContext, set/get_repo_sync_index
methods, set_repo_sync_index on Nip34WritePolicy, wiring in main.rs, and
the inline AcceptPurgatory registration block in builder.rs.

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