feat: Add two-tier rejected events index

Implements a sophisticated two-tier storage system for rejected repository
announcements to enable immediate re-processing when dependencies resolve.

## Architecture

**Tier 1: Hot Cache (2 minutes)**
- Stores full event objects for immediate re-processing
- Enables <1 second re-processing vs 24 hour wait
- Auto-expires to prevent memory growth
- Memory: ~200 KB typical, ~20 MB worst case

**Tier 2: Cold Index (7 days)**
- Stores metadata only (event_id, pubkey, identifier)
- Prevents repeated downloads of rejected events
- Enables invalidation when circumstances change
- Memory: ~1 MB typical

## Problem Solved

Without this system, maintainer announcements face a timing gap:

00:00 - Maintainer announcement rejected → Event discarded
00:02 - Owner announcement accepted (lists maintainer) → Want to re-process
00:02 - ❌ Maintainer announcement GONE → Must wait 24h for next sync

With two-tier system:

00:00 - Maintainer announcement rejected → Stored in both tiers
00:02 - Owner announcement accepted → Invalidate + get from hot cache
00:02 - ✅ Re-process immediately → Accepted in <1 second

## Implementation

New module: src/sync/rejected_index.rs
- RejectedEventsIndex: Public API combining both tiers
- HotCache: Internal struct for full event storage
- ColdIndex: Internal struct for metadata storage
- RejectionReason: Enum for tracking why events were rejected

Key methods:
- add_announcement(): Add to both tiers
- contains(): Check if event is rejected
- invalidate_and_get_events(): Remove from cold index, get from hot cache
- cleanup_expired(): Remove expired entries from both tiers

## Testing

9 comprehensive unit tests covering:
- Hot cache storage and retrieval
- Hot cache expiration
- Cold index metadata tracking
- Cold index invalidation
- Two-tier integration
- Cleanup of expired entries
- Hot cache misses after expiry
- Multiple maintainer repositories

All tests passing.

## Next Steps

PR2: Switch SyncManager to use new RejectedEventsIndex
PR3: Add invalidation + immediate re-processing logic
PR4: Add cleanup task + Prometheus metrics

Part of: Maintainer chain discovery fix
See: work/SOLUTION-SUMMARY-V2.md for full design

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